theorem Th148:
  variables_in Ex(x,y,H) = variables_in H \/ {x,y}
proof
  thus variables_in Ex(x,y,H) = variables_in Ex(y,H) \/ {x} by Th146
    .= variables_in H \/ {y} \/ {x} by Th146
    .= variables_in H \/ ({y} \/ {x}) by XBOOLE_1:4
    .= variables_in H \/ {x,y} by ENUMSET1:1;
end;
