docs: def _internal_sub(self, other, method=None):
          """ Used for specifing addition methods for
             __add__, __iadd__, __radd__
          """
          if hasattr(other, "datatype"):
              if other.datatype == self.datatype:
                  oval = other.value
              else:
                  oval = int(other.value)
          else:
              oval = int(other)
          if method == 'rsub':
              rtn_val = oval - self.value
          else:
              rtn_val = self.value - oval
          return XsdInteger(rtn_val)