theorem
  Free Ex(x,y,z,p) = Free p \ {x,y,z}
proof
  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;
end;
