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feat(library/data/set/basic): Added a supporting lemma for sUnion, which will be essential for proofs by induction on finite sets

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Jacob Gross 2015-11-26 11:18:51 -05:00 committed by Leonardo de Moura
parent 719a78d541
commit dd6bed371a
1 changed files with 20 additions and 0 deletions
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library/data/set/basic.lean
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@ -402,6 +402,26 @@ section
suppose x ∈ Union b,
obtain i (Hi : x ∈ b i), from this,
show x ∈ c, from H i Hi
theorem sUnion_insert (s : set (set X)) (a : set X) :
sUnion (insert a s) = a ∪ sUnion s :=
ext (take x, iff.intro
(suppose x ∈ sUnion (insert a s),
obtain c [(cias : c ∈ insert a s) (xc : x ∈ c)], from this,
or.elim cias
(suppose c = a,
show x ∈ a ∪ sUnion s, from or.inl (this ▸ xc))
(suppose c ∈ s,
show x ∈ a ∪ sUnion s, from or.inr (exists.intro c (and.intro this xc))))
(suppose x ∈ a ∪ sUnion s,
or.elim this
(suppose x ∈ a,
have a ∈ insert a s, from or.inl rfl,
show x ∈ sUnion (insert a s), from exists.intro a (and.intro this `x ∈ a`))
(suppose x ∈ sUnion s,
obtain c [(cs : c ∈ s) (xc : x ∈ c)], from this,
have c ∈ insert a s, from or.inr cs,
show x ∈ sUnion (insert a s), from exists.intro c (and.intro this `x ∈ c`))))
end
end set