theorem
   Sum <%c1,c2%> = c1 + c2
proof
  c1 in COMPLEX & c2 in COMPLEX by XCMPLX_0:def 2;
  then addcomplex "**" <%c1,c2%> = addcomplex.(c1,c2) by Th38
     .= c1+c2 by BINOP_2:def 3;
  hence thesis;
end;
