Give an LK proof of the sequent A $->$ B$,$ where

A $=$syn $$x$$y x $=$ fy

B $=$syn $$x$$y x $=$ ffy

Start by giving the specific instances of the LK equality axioms EL$1,...,$EL$5$ that you need in

your proof.

You may use abbreviations for formulas in your proof. You do not need to indicate weakinings

or exchanges.