Let p be given.
Assume Hp.
rewrite the current goal using Hdx (from left to right).
rewrite the current goal using Hdy (from left to right).
rewrite the current goal using Hdx2 (from left to right).
rewrite the current goal using Hdy2 (from left to right).
rewrite the current goal using Hdef (from left to right).
rewrite the current goal using Hsum (from left to right).
Use reflexivity.
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