theorem
  variables_in Ex(x,y,z,H) = variables_in H \/ {x,y,z}
proof
  thus variables_in Ex(x,y,z,H) = variables_in Ex(y,z,H) \/ {x} by Th146
    .= variables_in H \/ {y,z} \/ {x} by Th148
    .= variables_in H \/ ({y,z} \/ {x}) by XBOOLE_1:4
    .= variables_in H \/ {y,z,x} by ENUMSET1:3
    .= variables_in H \/ {x,y,z} by ENUMSET1:59;
end;
