theorem Th20:
  Sum Width <*M1,M2*>=width M1+width M2
proof
  thus Sum Width <*M1,M2*> = Sum ((Width <*M1*>)^(Width<*M2*>)) by Th18
    .= Sum Width <*M1*>+Sum Width<*M2*> by RVSUM_1:75
    .= width M1+Sum Width <*M2*> by Lm5
    .= width M1+width M2 by Lm5;
end;
