Using suffices in tactic

Stream: new members

[Ken Roe]

theorem testit (f:ℕ) (s:ℕ) :
    (f,s).fst=f :=
begin
   do {
       t ← target,
       trace t.to_raw_fmt,
       suffices xx:(1==2),admit
   },
end

Can someone correct the syntax of the above. I would like to use "suffices" rather than "change" for a tactic. I get the following error:

test.lean:13:26: error
invalid expression, 'by', 'begin', '{', or 'from' expected
test.lean:13:26: error
invalid suffices-expression, term
  admit
has type
  tactic unit : Type
but is expected to have type
  1 == 2 : Prop

[Kenny Lau]

@Ken Roe what are your imports?

[Ken Roe]

open tactic
open monad
open expr
open smt_tactic.

[Kenny Lau]

...

[Simon Hudon]

those are not imports, they are open statements