repeat match goal with
| [ |- context[wordToNat (natToWord ?sz ?w)] ] =>
let Heq := fresh "Heq" in
destruct (wordToNat_natToWord sz w) as [? [Heq ?]]; rewrite Heq
end.
match goal with
| [ |- context[wordToNat ?x + wordToNat ?y - ?x1 * pow2 ?sz + wordToNat ?z] ]
=> replace (wordToNat x + wordToNat y - x1 * pow2 sz + wordToNat z)
with (wordToNat x + wordToNat y + wordToNat z - x1 * pow2 sz) by auto
end.
match goal with
| [ |- context[wordToNat ?x + (wordToNat ?y + wordToNat ?z - ?x0 * pow2 ?sz)] ]
=> replace (wordToNat x + (wordToNat y + wordToNat z - x0 * pow2 sz))
with (wordToNat x + wordToNat y + wordToNat z - x0 * pow2 sz) by auto
end.
repeat rewrite drop_sub; auto.
Qed.

Theorem roundTrip_1 : forall sz, wordToNat (natToWord (S sz) 1) = 1.
induction sz; simpl in *; intuition.