coq - Software Foundations: apply ... with ... tactic -

i'm try run simple examples on apply ... ... tactic pierce's "software foundations".

it seems examples book doesn't work me:

theorem trans_eq: forall (x: type) (n m o: type),                     n = m -> m = o -> n = o. proof.   intros x n m o eq1 eq2. rewrite -> eq1. rewrite -> eq2. reflexivity. qed.  example trans_eq_example' : forall (a b c d e f : nat),      [a;b] = [c;d] ->      [c;d] = [e;f] ->      [a;b] = [e;f]. proof.   intros b c d e f eq1 eq2.   (* if tell coq apply trans_eq @ point,      can tell (by matching goal against      conclusion of lemma) should instantiate x      [nat], n [a,b], , o [e,f].      however, matching process doesn't determine      instantiation m: have supply 1 explicitly      adding (m:=[c,d]) invocation of      apply. *)   apply trans_eq (m:=[c;d]). apply eq1. apply eq2. qed. 

trans_eq_example' failed error:

trans_eq_example' < apply trans_eq (m:=[c;d]). toplevel input, characters 6-30: > apply trans_eq (m:=[c;d]). >       ^^^^^^^^^^^^^^^^^^^^^^^^ error: impossible unify "?1707 = ?1709" "[a; b] = [e; f]". 

additional information coq version:

coqtop -v coq proof assistant, version 8.4pl4 (july 2014) compiled on jul 27 2014 23:12:44 ocaml 4.01.0 

how can fix error?

the issue not apply typo in previous code. definition of trans_eq should be:

theorem trans_eq: forall (x:type) (n m o: x), n = m -> m = o -> n = o. 

note type of n m o should x, not type.