theorem Th16:
  Sum Len <*M1,M2*> = len M1 + len M2
proof
  thus Sum Len <*M1,M2*> = Sum ((Len <*M1*>)^(Len<*M2*>)) by Th14
    .= Sum Len <*M1*>+Sum Len<*M2*> by RVSUM_1:75
    .= len M1+Sum Len <*M2*> by Lm4
    .= len M1+len M2 by Lm4;
end;
