docs: def astype(self, dtype):
          """Return a point of the specified dtype."""
          if dtype == "int":
              return Point2D( int( round(self.x, 0) ), int( round(self.y, 0) ) )
          elif dtype == "float":
              return Point2D( float(self.x), float(self.y))
          else:
              raise(RuntimeError("Invalid dtype: {}".format(dtype)))