let p be ZF-formula; for x, y, z being Variable holds Free (Ex (x,y,z,p)) = (Free p) \ {x,y,z}
let x, y, z be Variable; Free (Ex (x,y,z,p)) = (Free p) \ {x,y,z}
thus Free (Ex (x,y,z,p)) =
(Free (Ex (y,z,p))) \ {x}
by Th66
.=
((Free p) \ {y,z}) \ {x}
by Th69
.=
(Free p) \ ({x} \/ {y,z})
by XBOOLE_1:41
.=
(Free p) \ {x,y,z}
by ENUMSET1:2
; verum