Goal:
    A : Type  t : A  P : A → A → Type  hd' : A  tl : list A  p : P t hd'
  ============================
   ((t = hd') + P t hd')%type


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := hd'
of type
intropattern
id0 := P
of type
intropattern
flags := 
of type
tauto_flags
X2 := A → Type
of type
constr of type tacvalue