code: def _call(self, x, out, **kwargs):
          
          # TODO: Implement zero padding
          if self.impl == 'numpy':
              out[:] = self._call_numpy(x.asarray())
          else:
              out[:] = self._call_pyfftw(x.asarray(), out.asarray(), **kwargs)
docs: Implement ``self(x, out[, **kwargs])``.
  
          Parameters
          ----------
          x : `domain` element
              Discretized function to be transformed
          out : `range` element
              Element to which the output is written
  
          Notes
          -----
          See the ``pyfftw_call`` function for ``**kwargs`` options.
          The parameters ``axes`` and ``halfcomplex`` cannot be
          overridden.
  
          See Also
          --------
          odl.trafos.backends.pyfftw_bindings.pyfftw_call :
              Call pyfftw backend directly