theorem Th34:
  Sum<*c1,c2*> = c1 + c2
proof
  reconsider s1=c1 as Element of COMPLEX by XCMPLX_0:def 2;
  thus Sum<*c1,c2*> = Sum<*s1*> + c2 by Th31
    .= c1 + c2 by FINSOP_1:11;
end;
