let p be ZF-formula; :: thesis: for x, y, z being Variable holds Free (All x,y,z,p) = (Free p) \ {x,y,z}
let x, y, z be Variable; :: thesis: Free (All x,y,z,p) = (Free p) \ {x,y,z}
thus Free (All x,y,z,p) = (Free (All y,z,p)) \ {x} by Th67
.= ((Free p) \ {y,z}) \ {x} by Th72
.= (Free p) \ ({x} \/ {y,z}) by XBOOLE_1:41
.= (Free p) \ {x,y,z} by ENUMSET1:42 ; :: thesis: verum