reserve z,z1,z2,z3,z4 for Element of F_Complex;

theorem
  z2 <> 0.F_Complex implies z1 / z2 = (-z1) / (-z2)
proof
  reconsider z19=z1,z29=z2 as Element of COMPLEX by Def1;
  assume
A1: z2 <> 0.F_Complex;
  then
A2: -z2 <> 0.F_Complex by VECTSP_1:28;
A3: -z19 = -z1 & -z29 = -z2 by Th2;
  thus z1 / z2 = z19 / z29 by A1,Th6
    .= (-z19) / (-z29) by XCMPLX_1:191
    .= (-z1) / (-z2) by A2,A3,Th6;
end;
