code: public ComplexDouble subi(ComplexDouble c, ComplexDouble result) {
      if (this == result) {
        r -= c.r;
        i -= c.i;
      } else {
        result.r = r - c.r;
        result.i = i - c.i;
      }
      return this;
    }
docs: Subtract two complex numbers, in-place
  
  @param c      complex number to subtract
  @param result resulting complex number
  @return same as result