diff --git a/library/data/finset/comb.lean b/library/data/finset/comb.lean
index d8ef9edf5..32a0282fc 100644
--- a/library/data/finset/comb.lean
+++ b/library/data/finset/comb.lean
@@ -111,61 +111,61 @@ ext (take y, iff.intro
         mem_image (mem_union_r xt) fxy)))
 end image
 
-/- filter and set-builder notation -/
-section filter
+/- separation and set-builder notation -/
+section sep
 variables {A : Type} [deceq : decidable_eq A]
 include deceq
 variables (p : A → Prop) [decp : decidable_pred p] (s : finset A) {x : A}
 include decp
 
-definition filter : finset A :=
+definition sep : finset A :=
 quot.lift_on s
   (λl, to_finset_of_nodup
     (list.filter p (subtype.elt_of l))
     (list.nodup_filter p (subtype.has_property l)))
   (λ l₁ l₂ u, quot.sound (perm.perm_filter u))
 
-notation [priority finset.prio] `{` binder ∈ s `|` r:(scoped:1 p, filter p s) `}` := r
+notation [priority finset.prio] `{` binder ∈ s `|` r:(scoped:1 p, sep p s) `}` := r
 
-theorem filter_empty : filter p ∅ = ∅ := rfl
+theorem sep_empty : sep p ∅ = ∅ := rfl
 
 variables {p s}
 
-theorem of_mem_filter : x ∈ filter p s → p x :=
+theorem of_mem_sep : x ∈ sep p s → p x :=
 quot.induction_on s (take l, list.of_mem_filter)
 
-theorem mem_of_mem_filter : x ∈ filter p s → x ∈ s :=
+theorem mem_of_mem_sep : x ∈ sep p s → x ∈ s :=
 quot.induction_on s (take l, list.mem_of_mem_filter)
 
-theorem mem_filter_of_mem {x : A} : x ∈ s → p x → x ∈ filter p s :=
+theorem mem_sep_of_mem {x : A} : x ∈ s → p x → x ∈ sep p s :=
 quot.induction_on s (take l, list.mem_filter_of_mem)
 
 variables (p s)
 
-theorem mem_filter_iff : x ∈ filter p s ↔ x ∈ s ∧ p x :=
+theorem mem_sep_iff : x ∈ sep p s ↔ x ∈ s ∧ p x :=
 iff.intro
-  (assume H, and.intro (mem_of_mem_filter H) (of_mem_filter H))
-  (assume H, mem_filter_of_mem (and.left H) (and.right H))
+  (assume H, and.intro (mem_of_mem_sep H) (of_mem_sep H))
+  (assume H, mem_sep_of_mem (and.left H) (and.right H))
 
-theorem mem_filter_eq : x ∈ filter p s = (x ∈ s ∧ p x) :=
-propext !mem_filter_iff
+theorem mem_sep_eq : x ∈ sep p s = (x ∈ s ∧ p x) :=
+propext !mem_sep_iff
 
 variable t : finset A
 
-theorem mem_filter_union_iff : x ∈ filter p (s ∪ t) ↔ x ∈ filter p s ∨ x ∈ filter p t :=
-by rewrite [*mem_filter_iff, mem_union_iff, and.right_distrib]
+theorem mem_sep_union_iff : x ∈ sep p (s ∪ t) ↔ x ∈ sep p s ∨ x ∈ sep p t :=
+by rewrite [*mem_sep_iff, mem_union_iff, and.right_distrib]
 
-end filter
+end sep
 
 section
 
 variables {A : Type} [deceqA : decidable_eq A]
 include deceqA
 
-theorem eq_filter_of_subset {s t : finset A} (ssubt : s ⊆ t) : s = {x ∈ t | x ∈ s} :=
+theorem eq_sep_of_subset {s t : finset A} (ssubt : s ⊆ t) : s = {x ∈ t | x ∈ s} :=
 ext (take x, iff.intro
-  (suppose x ∈ s, mem_filter_of_mem (mem_of_subset_of_mem ssubt this) this)
-  (suppose x ∈ {x ∈ t | x ∈ s}, of_mem_filter this))
+  (suppose x ∈ s, mem_sep_of_mem (mem_of_subset_of_mem ssubt this) this)
+  (suppose x ∈ {x ∈ t | x ∈ s}, of_mem_sep this))
 
 theorem mem_singleton_eq' (x a : A) : x ∈ '{a} = (x = a) :=
 by rewrite [mem_insert_eq, mem_empty_eq, or_false]
@@ -181,13 +181,13 @@ definition diff (s t : finset A) : finset A := {x ∈ s | x ∉ t}
 infix [priority finset.prio] `\`:70 := diff
 
 theorem mem_of_mem_diff {s t : finset A} {x : A} (H : x ∈ s \ t) : x ∈ s :=
-mem_of_mem_filter H
+mem_of_mem_sep H
 
 theorem not_mem_of_mem_diff {s t : finset A} {x : A} (H : x ∈ s \ t) : x ∉ t :=
-of_mem_filter H
+of_mem_sep H
 
 theorem mem_diff {s t : finset A} {x : A} (H1 : x ∈ s) (H2 : x ∉ t) : x ∈ s \ t :=
-mem_filter_of_mem H1 H2
+mem_sep_of_mem H1 H2
 
 theorem mem_diff_iff (s t : finset A) (x : A) : x ∈ s \ t ↔ x ∈ s ∧ x ∉ t :=
 iff.intro
diff --git a/library/data/finset/partition.lean b/library/data/finset/partition.lean
index f31bc5286..49de48afb 100644
--- a/library/data/finset/partition.lean
+++ b/library/data/finset/partition.lean
@@ -77,22 +77,22 @@ lemma binary_union (P : A → Prop) [decP : decidable_pred P] {S : finset A} :
   S = {a ∈ S | P a} ∪ {a ∈ S | ¬(P a)} :=
 ext take a, iff.intro
   (suppose a ∈ S, decidable.by_cases
-    (suppose P a, mem_union_l (mem_filter_of_mem `a ∈ S` this))
-    (suppose ¬ P a, mem_union_r (mem_filter_of_mem `a ∈ S` this)))
-  (suppose a ∈ filter P S ∪ {a ∈ S | ¬ P a}, or.elim (mem_or_mem_of_mem_union this)
-    (suppose a ∈ filter P S,      mem_of_mem_filter this)
-    (suppose a ∈ {a ∈ S | ¬ P a}, mem_of_mem_filter this))
+    (suppose P a, mem_union_l (mem_sep_of_mem `a ∈ S` this))
+    (suppose ¬ P a, mem_union_r (mem_sep_of_mem `a ∈ S` this)))
+  (suppose a ∈ sep P S ∪ {a ∈ S | ¬ P a}, or.elim (mem_or_mem_of_mem_union this)
+    (suppose a ∈ sep P S,      mem_of_mem_sep this)
+    (suppose a ∈ {a ∈ S | ¬ P a}, mem_of_mem_sep this))
 
 lemma binary_inter_empty {P : A → Prop} [decP : decidable_pred P] {S : finset A} :
   {a ∈ S | P a} ∩ {a ∈ S | ¬(P a)} = ∅ :=
-inter_eq_empty (take a, assume Pa nPa, absurd (of_mem_filter Pa) (of_mem_filter nPa))
+inter_eq_empty (take a, assume Pa nPa, absurd (of_mem_sep Pa) (of_mem_sep nPa))
 
 definition disjoint_sets (S : finset (finset A)) : Prop :=
   ∀ s₁ s₂ (P₁ : s₁ ∈ S) (P₂ : s₂ ∈ S), s₁ ≠ s₂ → s₁ ∩ s₂ = ∅
 
-lemma disjoint_sets_filter_of_disjoint_sets {P : finset A → Prop} [decP : decidable_pred P] {S : finset (finset A)} :
+lemma disjoint_sets_sep_of_disjoint_sets {P : finset A → Prop} [decP : decidable_pred P] {S : finset (finset A)} :
   disjoint_sets S → disjoint_sets {s ∈ S | P s} :=
-assume Pds, take s₁ s₂, assume P₁ P₂, Pds s₁ s₂ (mem_of_mem_filter P₁) (mem_of_mem_filter P₂)
+assume Pds, take s₁ s₂, assume P₁ P₂, Pds s₁ s₂ (mem_of_mem_sep P₁) (mem_of_mem_sep P₂)
 
 lemma binary_inter_empty_Union_disjoint_sets {P : finset A → Prop} [decP : decidable_pred P] {S : finset (finset A)} :
   disjoint_sets S → Union {s ∈ S | P s} id ∩ Union {s ∈ S | ¬P s} id = ∅ :=
@@ -100,8 +100,8 @@ assume Pds, inter_eq_empty (take a, assume Pa nPa,
   obtain s Psin Pains, from iff.elim_left !mem_Union_iff Pa,
   obtain t Ptin Paint, from iff.elim_left !mem_Union_iff nPa,
   assert s ≠ t,
-    from assume Peq, absurd (Peq ▸ of_mem_filter Psin) (of_mem_filter Ptin),
-  Pds s t (mem_of_mem_filter Psin) (mem_of_mem_filter Ptin) `s ≠ t` ▸ mem_inter Pains Paint)
+    from assume Peq, absurd (Peq ▸ of_mem_sep Psin) (of_mem_sep Ptin),
+  Pds s t (mem_of_mem_sep Psin) (mem_of_mem_sep Ptin) `s ≠ t` ▸ mem_inter Pains Paint)
 
 section
 variables {B: Type} [deceqB : decidable_eq B]
@@ -134,7 +134,7 @@ assume Pds, calc
         card (Union S id)
       = card (Union {s ∈ S | P s} id ∪ Union {s ∈ S | ¬P s} id) : binary_Union
   ... = card (Union {s ∈ S | P s} id) + card (Union {s ∈ S | ¬P s} id) : card_union_of_disjoint (binary_inter_empty_Union_disjoint_sets Pds)
-  ... = Sum {s ∈ S | P s} card + Sum {s ∈ S | ¬P s} card : by rewrite [*(card_Union_of_disjoint _ id (disjoint_sets_filter_of_disjoint_sets Pds))]
+  ... = Sum {s ∈ S | P s} card + Sum {s ∈ S | ¬P s} card : by rewrite [*(card_Union_of_disjoint _ id (disjoint_sets_sep_of_disjoint_sets Pds))]
 
 end partition
 end finset
diff --git a/library/data/finset/to_set.lean b/library/data/finset/to_set.lean
index 76700a3fd..d4472a5e6 100644
--- a/library/data/finset/to_set.lean
+++ b/library/data/finset/to_set.lean
@@ -52,10 +52,10 @@ theorem to_set_inter : ts (s ∩ t) = ts s ∩ ts t := funext (λ x, !mem_to_set
 theorem mem_to_set_diff : x ∈ s \ t = (x ∈ ts s \ ts t) := !mem_diff_eq
 theorem to_set_diff : ts (s \ t) = ts s \ ts t := funext (λ x, !mem_to_set_diff)
 
-theorem mem_to_set_filter (p : A → Prop) [h : decidable_pred p] : x ∈ filter p s = (x ∈ set.filter p s) :=
-  !finset.mem_filter_eq
-theorem to_set_filter (p : A → Prop) [h : decidable_pred p] : filter p s = set.filter p s :=
-  funext (λ x, !mem_to_set_filter)
+theorem mem_to_set_sep (p : A → Prop) [h : decidable_pred p] : x ∈ sep p s = (x ∈ set.sep p s) :=
+  !finset.mem_sep_eq
+theorem to_set_sep (p : A → Prop) [h : decidable_pred p] : sep p s = set.sep p s :=
+  funext (λ x, !mem_to_set_sep)
 
 theorem mem_to_set_image {B : Type} [h : decidable_eq B] (f : A → B) {s : finset A} {y : B} :
   y ∈ image f s = (y ∈ set.image f s) := !mem_image_eq
diff --git a/library/data/set/basic.lean b/library/data/set/basic.lean
index 25153cdbe..525c34f44 100644
--- a/library/data/set/basic.lean
+++ b/library/data/set/basic.lean
@@ -175,8 +175,8 @@ definition set_of (P : X → Prop) : set X := P
 notation `{` binder `|` r:(scoped:1 P, set_of P) `}` := r
 
 -- {x ∈ s | P}
-definition filter (P : X → Prop) (s : set X) : set X := λx, x ∈ s ∧ P x
-notation `{` binder ∈ s `|` r:(scoped:1 p, filter p s) `}` := r
+definition sep (P : X → Prop) (s : set X) : set X := λx, x ∈ s ∧ P x
+notation `{` binder ∈ s `|` r:(scoped:1 p, sep p s) `}` := r
 
 /- insert -/
 
@@ -188,9 +188,9 @@ notation `'{`:max a:(foldr `,` (x b, insert x b) ∅) `}`:0 := a
 theorem subset_insert (x : X) (a : set X) : a ⊆ insert x a :=
 take y, assume ys, or.inr ys
 
-/- filter -/
+/- separation -/
 
-theorem eq_filter_of_subset {s t : set X} (ssubt : s ⊆ t) : s = {x ∈ t | x ∈ s} :=
+theorem eq_sep_of_subset {s t : set X} (ssubt : s ⊆ t) : s = {x ∈ t | x ∈ s} :=
 setext (take x, iff.intro
   (suppose x ∈ s, and.intro (ssubt this) this)
   (suppose x ∈ {x ∈ t | x ∈ s}, and.right this))
diff --git a/library/data/set/finite.lean b/library/data/set/finite.lean
index 69c2720e6..abda72dc4 100644
--- a/library/data/set/finite.lean
+++ b/library/data/set/finite.lean
@@ -85,15 +85,15 @@ theorem to_finset_inter (s t : set A) [fins : finite s] [fint : finite t] :
   to_finset (s ∩ t) = (#finset to_finset s ∩ to_finset t) :=
 by apply to_finset_eq_of_to_set_eq; rewrite [finset.to_set_inter, *to_set_to_finset]
 
-theorem finite_filter [instance] (s : set A) (p : A → Prop) [h : decidable_pred p]
+theorem finite_sep [instance] (s : set A) (p : A → Prop) [h : decidable_pred p]
     [fins : finite s] :
   finite {x ∈ s | p x}  :=
-exists.intro (finset.filter p (to_finset s))
-  (by rewrite [finset.to_set_filter, *to_set_to_finset])
+exists.intro (finset.sep p (to_finset s))
+  (by rewrite [finset.to_set_sep, *to_set_to_finset])
 
-theorem to_finset_filter (s : set A) (p : A → Prop) [h : decidable_pred p] [fins : finite s] :
+theorem to_finset_sep (s : set A) (p : A → Prop) [h : decidable_pred p] [fins : finite s] :
   to_finset {x ∈ s | p x} = (#finset {x ∈ to_finset s | p x}) :=
-by apply to_finset_eq_of_to_set_eq; rewrite [finset.to_set_filter, to_set_to_finset]
+by apply to_finset_eq_of_to_set_eq; rewrite [finset.to_set_sep, to_set_to_finset]
 
 theorem finite_image [instance] {B : Type} [h : decidable_eq B] (f : A → B) (s : set A)
     [fins : finite s] :
@@ -107,14 +107,14 @@ theorem to_finset_image {B : Type} [h : decidable_eq B] (f : A → B) (s : set A
 by apply to_finset_eq_of_to_set_eq; rewrite [finset.to_set_image, to_set_to_finset]
 
 theorem finite_diff [instance] (s t : set A) [fins : finite s] : finite (s \ t) :=
-!finite_filter
+!finite_sep
 
 theorem to_finset_diff (s t : set A) [fins : finite s] [fint : finite t] :
   to_finset (s \ t) = (#finset to_finset s \ to_finset t) :=
 by apply to_finset_eq_of_to_set_eq; rewrite [finset.to_set_diff, *to_set_to_finset]
 
 theorem finite_subset {s t : set A} [fint : finite t] (ssubt : s ⊆ t) : finite s :=
-by rewrite (eq_filter_of_subset ssubt); apply finite_filter
+by rewrite (eq_sep_of_subset ssubt); apply finite_sep
 
 theorem to_finset_subset_to_finset_eq (s t : set A) [fins : finite s] [fint : finite t] :
   (#finset to_finset s ⊆ to_finset t) = (s ⊆ t) :=
diff --git a/library/theories/combinatorics/choose.lean b/library/theories/combinatorics/choose.lean
index 263fd2941..1458d0cc4 100644
--- a/library/theories/combinatorics/choose.lean
+++ b/library/theories/combinatorics/choose.lean
@@ -128,17 +128,17 @@ include deceqA
 private theorem aux₀ (s : finset A) : {t ∈ powerset s | card t = 0} = '{∅} :=
 ext (take t, iff.intro
   (assume H,
-    assert t = ∅, from eq_empty_of_card_eq_zero (of_mem_filter H),
+    assert t = ∅, from eq_empty_of_card_eq_zero (of_mem_sep H),
     show t ∈ '{ ∅ }, by rewrite [this, mem_singleton_eq'])
   (assume H,
     assert t = ∅, by rewrite mem_singleton_eq' at H; assumption,
-    by substvars; exact mem_filter_of_mem !empty_mem_powerset rfl))
+    by substvars; exact mem_sep_of_mem !empty_mem_powerset rfl))
 
 private theorem aux₁ (k : ℕ) : {t ∈ powerset (∅ : finset A) | card t = succ k} = ∅ :=
 eq_empty_of_forall_not_mem (take t, assume H,
-  assert t ∈ powerset ∅, from mem_of_mem_filter H,
+  assert t ∈ powerset ∅, from mem_of_mem_sep H,
   assert t = ∅, by rewrite [powerset_empty at this, mem_singleton_eq' at this]; assumption,
-  have card (∅ : finset A) = succ k, by rewrite -this; apply of_mem_filter H,
+  have card (∅ : finset A) = succ k, by rewrite -this; apply of_mem_sep H,
   nat.no_confusion this)
 
 private theorem aux₂ {a : A} {s t : finset A} (anins : a ∉ s) (tpows : t ∈ powerset s) : a ∉ t :=
@@ -160,11 +160,11 @@ iff.intro
         anins this),
     assert t' = erase a t, by rewrite [-teq, erase_insert (aux₂ anins t'pows)],
     have card t' = k, by rewrite [this, card_erase_of_mem aint, cardt],
-    mem_image (mem_filter_of_mem t'pows this) teq)
+    mem_image (mem_sep_of_mem t'pows this) teq)
   (assume H,
     obtain t' [Ht' (teq : insert a t' = t)], from exists_of_mem_image H,
-    assert t'pows : t' ∈ powerset s, from mem_of_mem_filter Ht',
-    assert cardt' : card t' = k, from of_mem_filter Ht',
+    assert t'pows : t' ∈ powerset s, from mem_of_mem_sep Ht',
+    assert cardt' : card t' = k, from of_mem_sep Ht',
     and.intro
       (show t ∈ (insert a) '[powerset s], from mem_image t'pows teq)
       (show card t = succ k,
@@ -175,7 +175,7 @@ private theorem aux₄ {a : A} {s : finset A} (anins : a ∉ s) (k : ℕ) :
     {t ∈ powerset s | card t = succ k} ∪ (insert a) '[{t ∈ powerset s | card t = k}] :=
 begin
   apply ext, intro t,
-  rewrite [powerset_insert anins, mem_union_iff, *mem_filter_iff, mem_union_iff, and.right_distrib,
+  rewrite [powerset_insert anins, mem_union_iff, *mem_sep_iff, mem_union_iff, and.right_distrib,
            aux₃ anins]
 end
 
@@ -183,7 +183,7 @@ private theorem aux₅ {a : A} {s : finset A} (anins : a ∉ s) (k : ℕ) :
   {t ∈ powerset s | card t = succ k} ∩ (insert a) '[{t ∈ powerset s | card t = k}] = ∅ :=
 inter_eq_empty
   (take t, assume Ht₁ Ht₂,
-    have tpows : t ∈ powerset s, from mem_of_mem_filter Ht₁,
+    have tpows : t ∈ powerset s, from mem_of_mem_sep Ht₁,
     have anint : a ∉ t, from aux₂ anins tpows,
     obtain t' [Ht' (teq : insert a t' = t)], from exists_of_mem_image Ht₂,
     have aint : a ∈ t, by rewrite -teq; apply mem_insert,
@@ -194,9 +194,9 @@ private theorem aux₆ {a : A} {s : finset A} (anins : a ∉ s) (k : ℕ) :
 have set.inj_on (insert a) (ts {t ∈ powerset s| card t = k}), from
   take t₁ t₂, assume Ht₁ Ht₂,
   assume Heq : insert a t₁ = insert a t₂,
-  have t₁ ∈ powerset s, from mem_of_mem_filter Ht₁,
+  have t₁ ∈ powerset s, from mem_of_mem_sep Ht₁,
   assert anint₁ : a ∉ t₁, from aux₂ anins this,
-  have t₂ ∈ powerset s, from mem_of_mem_filter Ht₂,
+  have t₂ ∈ powerset s, from mem_of_mem_sep Ht₂,
   assert anint₂ : a ∉ t₂, from aux₂ anins this,
   calc
     t₁    = erase a (insert a t₁) : by rewrite (erase_insert anint₁)
diff --git a/library/theories/group_theory/action.lean b/library/theories/group_theory/action.lean
index afd09c1a5..5985aada4 100644
--- a/library/theories/group_theory/action.lean
+++ b/library/theories/group_theory/action.lean
@@ -72,11 +72,11 @@ lemma is_fixed_point_of_mem_fixed_points :
   a ∈ fixed_points hom H → is_fixed_point hom H a :=
 assume Pain, take h, assume Phin,
   eq_of_mem_singleton
-    (of_mem_filter Pain ▸ orbit_of_exists (exists.intro h (and.intro Phin rfl)))
+    (of_mem_sep Pain ▸ orbit_of_exists (exists.intro h (and.intro Phin rfl)))
 
 lemma mem_fixed_points_of_exists_of_is_fixed_point :
   (∃ h, h ∈ H) → is_fixed_point hom H a → a ∈ fixed_points hom H :=
-assume Pex Pfp, mem_filter_of_mem !mem_univ
+assume Pex Pfp, mem_sep_of_mem !mem_univ
   (ext take x, iff.intro
     (assume Porb, obtain h Phin Pha, from exists_of_orbit Porb,
       by rewrite [mem_singleton_eq, -Pha, Pfp h Phin])
@@ -122,19 +122,19 @@ rfl
 
 lemma stab_lmul {f g : G} : g ∈ stab hom H a → hom (f*g) a = hom f a :=
 assume Pgstab,
-assert hom g a = a, from of_mem_filter Pgstab, calc
+assert hom g a = a, from of_mem_sep Pgstab, calc
   hom (f*g) a = perm.f ((hom f) * (hom g)) a : is_hom hom
           ... = ((hom f) ∘ (hom g)) a        : by rewrite perm_f_mul
           ... = (hom f) a                    : by unfold compose; rewrite this
 
 lemma stab_subset : stab hom H a ⊆ H :=
       begin
-        apply subset_of_forall, intro f Pfstab, apply mem_of_mem_filter Pfstab
+        apply subset_of_forall, intro f Pfstab, apply mem_of_mem_sep Pfstab
       end
 
 lemma reverse_move {h g : G} : g ∈ moverset hom H a (hom h a) → hom (h⁻¹*g) a = a :=
 assume Pg,
-assert hom g a = hom h a, from of_mem_filter Pg, calc
+assert hom g a = hom h a, from of_mem_sep Pg, calc
   hom (h⁻¹*g) a = perm.f ((hom h⁻¹) * (hom g)) a : by rewrite (is_hom hom)
   ... = ((hom h⁻¹) ∘ hom g) a                    : by rewrite perm_f_mul
   ... = perm.f ((hom h)⁻¹ * hom h) a             : by unfold compose; rewrite [this, perm_f_mul, hom_map_inv hom h]
@@ -145,11 +145,11 @@ lemma moverset_inj_on_orbit : set.inj_on (moverset hom H a) (ts (orbit hom H a))
       take b1 b2,
       assume Pb1, obtain h1 Ph1₁ Ph1₂, from exists_of_orbit Pb1,
       assert Ph1b1 : h1 ∈ moverset hom H a b1,
-        from mem_filter_of_mem Ph1₁ Ph1₂,
+        from mem_sep_of_mem Ph1₁ Ph1₂,
       assume Psetb2 Pmeq, begin
         subst b1,
         rewrite Pmeq at Ph1b1,
-        apply of_mem_filter Ph1b1
+        apply of_mem_sep Ph1b1
       end
 
 variable [subgH : is_finsubg H]
@@ -161,37 +161,37 @@ lemma subg_stab_of_move {h g : G} :
       assert Phinvg : h⁻¹*g ∈ H, from begin
         apply finsubg_mul_closed H,
           apply finsubg_has_inv H, assumption,
-          apply mem_of_mem_filter Pg
+          apply mem_of_mem_sep Pg
         end,
-      mem_filter_of_mem Phinvg (reverse_move Pg)
+      mem_sep_of_mem Phinvg (reverse_move Pg)
 
 lemma subg_stab_closed : finset_mul_closed_on (stab hom H a) :=
-      take f g, assume Pfstab, assert Pf : hom f a = a, from of_mem_filter Pfstab,
+      take f g, assume Pfstab, assert Pf : hom f a = a, from of_mem_sep Pfstab,
       assume Pgstab,
       assert Pfg : hom (f*g) a = a, from calc
         hom (f*g) a = (hom f) a : stab_lmul Pgstab
         ... = a : Pf,
       assert PfginH : (f*g) ∈ H,
-        from finsubg_mul_closed H (mem_of_mem_filter Pfstab) (mem_of_mem_filter Pgstab),
-      mem_filter_of_mem PfginH Pfg
+        from finsubg_mul_closed H (mem_of_mem_sep Pfstab) (mem_of_mem_sep Pgstab),
+      mem_sep_of_mem PfginH Pfg
 
 lemma subg_stab_has_one : 1 ∈ stab hom H a :=
       assert P : hom 1 a = a, from calc
         hom 1 a = perm.f (1 : perm S) a : {hom_map_one hom}
         ... = a                         : rfl,
       assert PoneinH : 1 ∈ H, from finsubg_has_one H,
-      mem_filter_of_mem PoneinH P
+      mem_sep_of_mem PoneinH P
 
 lemma subg_stab_has_inv : finset_has_inv (stab hom H a) :=
-      take f, assume Pfstab, assert Pf : hom f a = a, from of_mem_filter Pfstab,
+      take f, assume Pfstab, assert Pf : hom f a = a, from of_mem_sep Pfstab,
       assert Pfinv : hom f⁻¹ a = a, from calc
         hom f⁻¹ a = hom f⁻¹ ((hom f) a)      : by rewrite Pf
         ... = perm.f ((hom f⁻¹) * (hom f)) a : by rewrite perm_f_mul
         ... = hom (f⁻¹ * f) a                : by rewrite (is_hom hom)
         ... = hom 1 a                        : by rewrite mul.left_inv
         ... = perm.f (1 : perm S) a          : by rewrite (hom_map_one hom),
-      assert PfinvinH : f⁻¹ ∈ H, from finsubg_has_inv H (mem_of_mem_filter Pfstab),
-      mem_filter_of_mem PfinvinH Pfinv
+      assert PfinvinH : f⁻¹ ∈ H, from finsubg_has_inv H (mem_of_mem_sep Pfstab),
+      mem_sep_of_mem PfinvinH Pfinv
 
 definition subg_stab_is_finsubg [instance] :
            is_finsubg (stab hom H a) :=
@@ -201,17 +201,17 @@ lemma subg_lcoset_eq_moverset {h : G} :
       h ∈ H → fin_lcoset (stab hom H a) h = moverset hom H a (hom h a) :=
       assume Ph, ext (take g, iff.intro
       (assume Pl, obtain f (Pf₁ : f ∈ stab hom H a) (Pf₂ : h*f = g), from exists_of_mem_image Pl,
-       assert Pfstab : hom f a = a, from of_mem_filter Pf₁,
+       assert Pfstab : hom f a = a, from of_mem_sep Pf₁,
        assert PginH : g ∈ H, begin
         subst Pf₂,
         apply finsubg_mul_closed H,
           assumption,
-          apply mem_of_mem_filter Pf₁
+          apply mem_of_mem_sep Pf₁
         end,
       assert Pga : hom g a = hom h a, from calc
         hom g a = hom (h*f) a : by subst g
         ... = hom h a         : stab_lmul Pf₁,
-      mem_filter_of_mem PginH Pga)
+      mem_sep_of_mem PginH Pga)
       (assume Pr, begin
        rewrite [↑fin_lcoset, mem_image_iff],
        existsi h⁻¹*g,
@@ -333,24 +333,24 @@ iff.elim_left (exists_iff_mem_orbits orb)
 lemma fixed_point_orbits_eq : fixed_point_orbits hom H = image (orbit hom H) (fixed_points hom H) :=
 ext take s, iff.intro
   (assume Pin,
-   obtain Psin Ps, from iff.elim_left !mem_filter_iff Pin,
+   obtain Psin Ps, from iff.elim_left !mem_sep_iff Pin,
    obtain a Pa, from exists_of_mem_orbits Psin,
    mem_image
-     (mem_filter_of_mem !mem_univ (eq.symm
+     (mem_sep_of_mem !mem_univ (eq.symm
        (eq_of_card_eq_of_subset (by rewrite [card_singleton, Pa, Ps])
          (subset_of_forall
            take x, assume Pxin, eq_of_mem_singleton Pxin ▸ in_orbit_refl))))
      Pa)
   (assume Pin,
    obtain a Pain Porba, from exists_of_mem_image Pin,
-   mem_filter_of_mem
+   mem_sep_of_mem
      (begin esimp [orbits, equiv_classes, orbit_partition], rewrite [mem_image_iff],
        existsi a, exact and.intro !mem_univ Porba end)
-     (begin substvars, rewrite [of_mem_filter Pain] end))
+     (begin substvars, rewrite [of_mem_sep Pain] end))
 
 lemma orbit_inj_on_fixed_points : set.inj_on (orbit hom H) (ts (fixed_points hom H)) :=
 take a₁ a₂, begin
-  rewrite [-*mem_eq_mem_to_set, ↑fixed_points, *mem_filter_iff],
+  rewrite [-*mem_eq_mem_to_set, ↑fixed_points, *mem_sep_iff],
   intro Pa₁ Pa₂,
   rewrite [and.right Pa₁, and.right Pa₂],
   exact eq_of_singleton_eq
@@ -363,7 +363,7 @@ lemma orbit_class_equation : card S = Sum (orbits hom H) card :=
 class_equation (orbit_partition hom H)
 
 lemma card_fixed_point_orbits : Sum (fixed_point_orbits hom H) card = card (fixed_point_orbits hom H) :=
-calc Sum _ _ = Sum (fixed_point_orbits hom H) (λ x, 1) : Sum_ext (take c Pin, of_mem_filter Pin)
+calc Sum _ _ = Sum (fixed_point_orbits hom H) (λ x, 1) : Sum_ext (take c Pin, of_mem_sep Pin)
          ... = card (fixed_point_orbits hom H) * 1 : Sum_const_eq_card_mul
          ... = card (fixed_point_orbits hom H) : mul_one (card (fixed_point_orbits hom H))
 
@@ -448,7 +448,7 @@ subset_of_forall take g, begin
   intro Pg,
   rewrite -Pg at PH,
   apply finsubg_has_inv,
-  apply mem_filter_of_mem !mem_univ,
+  apply mem_sep_of_mem !mem_univ,
   intro h Ph,
   assert Phg : fin_lcoset (fin_lcoset H g) h = fin_lcoset H g, exact PH Ph,
   revert Phg,
@@ -536,9 +536,9 @@ ext (take (pp : perm (fin (succ n))), iff.intro
     pp maxi = lift_perm p maxi : {eq.symm Pp}
         ... = lift_fun p maxi : rfl
         ... = maxi : lift_fun_max,
-  mem_filter_of_mem !mem_univ Ppp)
+  mem_sep_of_mem !mem_univ Ppp)
   (assume Pstab,
-  assert Ppp : pp maxi = maxi, from of_mem_filter Pstab,
+  assert Ppp : pp maxi = maxi, from of_mem_sep Pstab,
   mem_image !mem_univ (lift_lower_eq Ppp)))
 
 definition move_from_max_to (i : fin (succ n)) : perm (fin (succ n)) :=
diff --git a/library/theories/group_theory/cyclic.lean b/library/theories/group_theory/cyclic.lean
index ce3c7b36e..5e26742ec 100644
--- a/library/theories/group_theory/cyclic.lean
+++ b/library/theories/group_theory/cyclic.lean
@@ -96,7 +96,7 @@ card_le_card_of_subset !subset_univ
 
 lemma cyc_has_one (a : A) : 1 ∈ cyc a :=
 begin
-  apply mem_filter_of_mem !mem_univ,
+  apply mem_sep_of_mem !mem_univ,
   existsi 0, apply and.intro,
     apply zero_lt_succ,
     apply pow_zero
@@ -108,11 +108,11 @@ length_pos_of_mem (cyc_has_one a)
 lemma cyc_mul_closed (a : A) : finset_mul_closed_on (cyc a) :=
 take g h, assume Pgin Phin,
 obtain n Plt Pe, from exists_pow_eq_one a,
-obtain i Pilt Pig, from of_mem_filter Pgin,
-obtain j Pjlt Pjh, from of_mem_filter Phin,
+obtain i Pilt Pig, from of_mem_sep Pgin,
+obtain j Pjlt Pjh, from of_mem_sep Phin,
 begin
   rewrite [-Pig, -Pjh, -pow_add, pow_mod Pe],
-  apply mem_filter_of_mem !mem_univ,
+  apply mem_sep_of_mem !mem_univ,
   existsi ((i + j) mod (succ n)), apply and.intro,
     apply nat.lt.trans (mod_lt (i+j) !zero_lt_succ) (succ_lt_succ Plt),
     apply rfl
@@ -121,20 +121,20 @@ end
 lemma cyc_has_inv (a : A) : finset_has_inv (cyc a) :=
 take g, assume Pgin,
 obtain n Plt Pe, from exists_pow_eq_one a,
-obtain i Pilt Pig, from of_mem_filter Pgin,
+obtain i Pilt Pig, from of_mem_sep Pgin,
 let ni := -(mk_mod n i) in
 assert Pinv : g*a^ni = 1, by
   rewrite [-Pig, mk_pow_mod Pe, -(pow_madd Pe), add.right_inv],
 begin
   rewrite [inv_eq_of_mul_eq_one Pinv],
-  apply mem_filter_of_mem !mem_univ,
+  apply mem_sep_of_mem !mem_univ,
   existsi ni, apply and.intro,
     apply nat.lt.trans (is_lt ni) (succ_lt_succ Plt),
     apply rfl
 end
 
 lemma self_mem_cyc (a : A) : a ∈ cyc a :=
-mem_filter_of_mem !mem_univ
+mem_sep_of_mem !mem_univ
   (exists.intro (1 : nat) (and.intro (succ_lt_succ card_pos) !pow_one))
 
 lemma mem_cyc (a : A) : ∀ {n : nat}, a^n ∈ cyc a
@@ -145,7 +145,7 @@ lemma mem_cyc (a : A) : ∀ {n : nat}, a^n ∈ cyc a
 lemma order_le {a : A} {n : nat} : a^(succ n) = 1 → order a ≤ succ n :=
 assume Pe, let s := image (pow a) (upto (succ n)) in
 assert Psub: cyc a ⊆ s, from subset_of_forall
-  (take g, assume Pgin, obtain i Pilt Pig, from of_mem_filter Pgin, begin
+  (take g, assume Pgin, obtain i Pilt Pig, from of_mem_sep Pgin, begin
   rewrite [-Pig, pow_mod Pe],
   apply mem_image,
     apply mem_upto_of_lt (mod_lt i !zero_lt_succ),
diff --git a/library/theories/group_theory/finsubg.lean b/library/theories/group_theory/finsubg.lean
index fb09c39c9..7074531ff 100644
--- a/library/theories/group_theory/finsubg.lean
+++ b/library/theories/group_theory/finsubg.lean
@@ -355,7 +355,7 @@ variable [finsubgH : is_finsubg H]
 include finsubgH
 
 lemma subset_normalizer : H ⊆ normalizer H :=
-subset_of_forall take g, assume PginH, mem_filter_of_mem !mem_univ
+subset_of_forall take g, assume PginH, mem_sep_of_mem !mem_univ
   (take h, assume PhinH, finsubg_conj_closed PginH PhinH)
 
 lemma normalizer_has_one : 1 ∈ normalizer H :=
@@ -363,10 +363,10 @@ mem_of_subset_of_mem subset_normalizer (finsubg_has_one H)
 
 lemma normalizer_mul_closed : finset_mul_closed_on (normalizer H) :=
 take f g, assume Pfin Pgin,
-mem_filter_of_mem !mem_univ take h, assume Phin, begin
+mem_sep_of_mem !mem_univ take h, assume Phin, begin
   rewrite [-conj_compose],
-  apply of_mem_filter Pfin,
-  apply of_mem_filter Pgin,
+  apply of_mem_sep Pfin,
+  apply of_mem_sep Pgin,
   exact Phin
 end
 
@@ -375,11 +375,11 @@ assume Pgin,
 eq_of_card_eq_of_subset (card_image_eq_of_inj_on (take h j, assume P1 P2, !conj_inj))
   (subset_of_forall take h, assume Phin,
   obtain j Pjin Pj, from exists_of_mem_image Phin,
-  begin substvars, apply of_mem_filter Pgin, exact Pjin end)
+  begin substvars, apply of_mem_sep Pgin, exact Pjin end)
 
 lemma normalizer_has_inv : finset_has_inv (normalizer H) :=
 take g, assume Pgin,
-mem_filter_of_mem !mem_univ take h, begin
+mem_sep_of_mem !mem_univ take h, begin
   rewrite [-(conj_eq_of_mem_normalizer Pgin) at {1}, mem_image_iff],
   intro Pex, cases Pex with k Pk,
   rewrite [-(and.right Pk), conj_compose, mul.left_inv, conj_id],
@@ -401,11 +401,11 @@ lemma lrcoset_same_of_mem_normalizer {g : G} :
   g ∈ normalizer H → fin_lcoset H g = fin_rcoset H g :=
 assume Pg, ext take h, iff.intro
   (assume Pl, obtain j Pjin Pj, from exists_of_mem_image Pl,
-  mem_image (of_mem_filter Pg j Pjin)
+  mem_image (of_mem_sep Pg j Pjin)
     (calc g*j*g⁻¹*g = g*j : inv_mul_cancel_right
                 ... = h   : Pj))
   (assume Pr, obtain j Pjin Pj, from exists_of_mem_image Pr,
-  mem_image (of_mem_filter (finsubg_has_inv (normalizer H) Pg) j Pjin)
+  mem_image (of_mem_sep (finsubg_has_inv (normalizer H) Pg) j Pjin)
     (calc g*(g⁻¹*j*g⁻¹⁻¹) = g*(g⁻¹*j*g)   : inv_inv
                       ... = g*(g⁻¹*(j*g)) : mul.assoc
                       ... = j*g           : mul_inv_cancel_left
diff --git a/library/theories/group_theory/pgroup.lean b/library/theories/group_theory/pgroup.lean
index 0dbce534e..5389ec245 100644
--- a/library/theories/group_theory/pgroup.lean
+++ b/library/theories/group_theory/pgroup.lean
@@ -35,7 +35,7 @@ lemma card_mod_eq_of_action_by_psubg {p : nat} :
   rewrite [@orbit_class_equation' G S ambientG finS deceqS hom Hom H subgH],
   apply add_mod_eq_of_dvd, apply dvd_Sum_of_dvd,
   intro s Psin,
-  rewrite mem_filter_iff at Psin,
+  rewrite mem_sep_iff at Psin,
   cases Psin with Psinorbs Pcardne,
   esimp [orbits, equiv_classes, orbit_partition] at Psinorbs,
   rewrite mem_image_iff at Psinorbs,
diff --git a/library/theories/number_theory/prime_factorization.lean b/library/theories/number_theory/prime_factorization.lean
index 596a572c0..fbb053c7f 100644
--- a/library/theories/number_theory/prime_factorization.lean
+++ b/library/theories/number_theory/prime_factorization.lean
@@ -220,15 +220,15 @@ dvd.antisymm
 definition prime_factors (n : ℕ) : finset ℕ := { p ∈ upto (succ n) | prime p ∧ p ∣ n }
 
 theorem prime_of_mem_prime_factors {p n : ℕ} (H : p ∈ prime_factors n) : prime p :=
-and.left (of_mem_filter H)
+and.left (of_mem_sep H)
 
 theorem dvd_of_mem_prime_factors {p n : ℕ} (H : p ∈ prime_factors n) : p ∣ n :=
-and.right (of_mem_filter H)
+and.right (of_mem_sep H)
 
 theorem mem_prime_factors {p n : ℕ} (npos : n > 0) (primep : prime p) (pdvdn : p ∣ n) :
   p ∈ prime_factors n :=
 have plen : p ≤ n, from le_of_dvd npos pdvdn,
-mem_filter_of_mem (mem_upto_of_lt (lt_succ_of_le plen)) (and.intro primep pdvdn)
+mem_sep_of_mem (mem_upto_of_lt (lt_succ_of_le plen)) (and.intro primep pdvdn)
 
 /- prime factorization -/