reserve a, b, c, d, e for Complex;

theorem :: REAL_2'67_1
  a / b / c = a / c / b
proof
  thus a/b/c =a*b"/c by XCMPLX_0:def 9
    .=a*b"*c" by XCMPLX_0:def 9
    .=a*c"*b"
    .=a/c*b" by XCMPLX_0:def 9
    .=a/c/b by XCMPLX_0:def 9;
end;
