code: def _call(self, x, out=None):
          
          if self.domain == self.range:
              if out is None:
                  out = 0 * x
              else:
                  out.lincomb(0, x)
          else:
              result = self.range.zero()
              if out is None:
                  out = result
              else:
                  out.assign(result)
          return out
docs: Return the zero vector or assign it to ``out``.