theorem
  <*c1*> + <*c2*> = <*c1+c2*>
proof
  reconsider s1=c1, s2=c2 as Element of COMPLEX by XCMPLX_0:def 2;
  <*s1*> + <*s2*> = <*addcomplex.(s1,s2)*> by FINSEQ_2:74
    .= <*c1+c2*> by BINOP_2:def 3;
  hence thesis;
end;
