I was mildly surprised to see a heart in dummy, but had no worries-- trusty partner would give me trump suit preference! A high spade would show the ace of hearts, and a low spade would show a singleton diamond.
Declarer won in hand and led the queen of spades. I jumped with the ace, and partner played... the four! Thrilled with my chain of reasoning, I led another diamond. The whole hand, of course, was: