urec.lean:3:0: error: invalid user defined recursor, result type must be of the form (C i t), where C and t are bound variables, and i is a (possibly empty) sequence of bound variables
urec.lean:5:0: error: invalid user defined recursor, 'nat.rec' is a builtin recursor
urec.lean:19:0: error: invalid user defined recursor, motive 'C' must have a type of the form (C : Pi (i : B A), I A i -> Type), where A is (possibly empty) sequence of bound variables (aka parameters), (i : B A) is a (possibly empty) telescope (aka indices), and I is a constant
urec.lean:23:0: error: invalid user defined recursor, resultant type of minor premise 'a' must be of the form (M ...)
myrec.{l_1 l_2} :
  Π (A : Type.{l_1}) (M : list.{l_1} A → Type.{l_2}) (l : list.{l_1} A),
    M (@nil.{l_1} A) → (Π (a : A), M [a]) → (Π (a₁ a₂ : A), M [a₁, a₂]) → M l
recursor information
  num. parameters:    1
  num. indices:       0
  motive univ. pos. : 1
  motive pos.:        1
  major premise pos.: 2
  dep. elimination:   1
recursor information
  num. parameters:    0
  num. indices:       0
  recursor eliminate only to Prop
  motive pos.:        0
  major premise pos.: 1
  dep. elimination:   1
vector.induction_on.{l_1} :
  ∀ {A : Type.{l_1}} {C : Π (a : nat), vector.{l_1} A a → Prop} {a : nat} (n : vector.{l_1} A a),
    C nat.zero (@vector.nil.{l_1} A) →
    (∀ {n : nat} (a : A) (a_1 : vector.{l_1} A n), C n a_1 → C (nat.succ n) (@vector.cons.{l_1} A n a a_1)) →
    C a n
recursor information
  num. parameters:    1
  num. indices:       1
  recursor eliminate only to Prop
  motive pos.:        1
  major premise pos.: 3
  dep. elimination:   1
Exists.rec.{l_1} :
  ∀ {A : Type.{l_1}} {P : A → Prop} {C : Prop},
    (∀ (a : A), P a → C) → @Exists.{l_1} A P → C
recursor information
  num. parameters:    2
  num. indices:       0
  recursor eliminate only to Prop
  motive pos.:        2
  major premise pos.: 4
  dep. elimination:   0