def is_space : char → Prop
| #" "    := true
| #"\x09" := true -- \t
| #"\n"   := true
| #"\x0d" := true -- \r
| _       := false

instance is_space.decidable_pred : decidable_pred is_space :=
begin delta is_space, apply_instance end

def f (a : nat) : nat :=
a + 2

open tactic

meta def check_target (p : pexpr) : tactic unit :=
do t ← target, e ← to_expr p, guard (expr.alpha_eqv t e)

lemma flemma : f 0 = 2 :=
begin
  delta f,
  check_target `(0 + 2 = 2),
  reflexivity
end