theorem Th65:
  -(z1 + z2) = -z1 + -z2
proof
  (z1 + z2) + (-z1 + -z2) = z2 + z1 + -z1 + -z2 by FINSEQOP:28
    .= z2 + (z1 + -z1) + -z2 by FINSEQOP:28
    .= z2 + 0c n + -z2 by Th51,Th52,BINOP_2:1,FINSEQOP:73
    .= z2 + -z2 by BINOP_2:1,FINSEQOP:56
    .= 0c n by Th51,Th52,BINOP_2:1,FINSEQOP:73;
  hence thesis by Th51,Th52,BINOP_2:1,FINSEQOP:74;
end;
