theorem Th60:
  k in Seg n & c = z.k implies (-z).k = -c
proof
  assume that
A1: k in Seg n and
A2: c = z.k;
  k in dom(-z) by A1,Lm4;
  hence (-z).k = compcomplex.c by A2,FUNCT_1:12
    .= -c by BINOP_2:def 1;
end;
