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

theorem
  z <> 0.F_Complex implies z1 = (z1 * z) / z
proof
  reconsider z19=z1,z9=z as Element of COMPLEX by Def1;
  assume
A1: z <> 0.F_Complex;
  hence z1 = (z19 * z9) / z9 by Th7,XCMPLX_1:89
    .= (z1 * z) / z by A1,Th6;
end;
