Goal:
  
  A : Type
  p : A → bool
  q : A → bool
  l : list A
  H0 : ∀ x : A, In x l → p x → q x
  x : A
  H2 : In x l
  H1 : p x = true
  ============================
   (In x l)


DEBUG:Going to execute:
solve
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]