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

theorem
  z1 <> 0.F_Complex & z2 <> 0.F_Complex implies (z1 / z2)" = z2 / z1
proof
  reconsider z19=z1,z29=z2 as Element of COMPLEX by Def1;
  assume
A1: z1 <> 0.F_Complex;
  assume
A2: z2 <> 0.F_Complex;
  then
A3: z19 / z29 = z1 / z2 by Th6;
  z1 / z2 <> 0.F_Complex by A1,A2,Th26;
  hence (z1 / z2)" = (z19 / z29)" by A3,Th5
    .= z29 / z19 by XCMPLX_1:213
    .= z2 / z1 by A1,Th6;
end;
