theorem
  variables_in H1 'or' H2 = variables_in H1 \/ variables_in H2
proof
  thus variables_in H1 'or' H2 = variables_in 'not' H1 '&' 'not' H2 by Th140
    .= variables_in 'not' H1 \/ variables_in 'not' H2 by Th141
    .= variables_in H1 \/ variables_in 'not' H2 by Th140
    .= variables_in H1 \/ variables_in H2 by Th140;
end;
