let p be ZF-formula; :: thesis: for x, y being Variable holds Free (Ex x,y,p) = (Free p) \ {x,y}
let x, y be Variable; :: thesis: Free (Ex x,y,p) = (Free p) \ {x,y}
thus Free (Ex x,y,p) = (Free (Ex y,p)) \ {x} by Th71
.= ((Free p) \ {y}) \ {x} by Th71
.= (Free p) \ ({x} \/ {y}) by XBOOLE_1:41
.= (Free p) \ {x,y} by ENUMSET1:41 ; :: thesis: verum