theorem Th146:
  variables_in Ex(x,H) = variables_in H \/ {x}
proof
  thus variables_in Ex(x,H) = variables_in All(x,'not' H) by Th140
    .= variables_in 'not' H \/ {x} by Th142
    .= variables_in H \/ {x} by Th140;
end;
