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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _call(self, x):
 """Return the group L1-norm of ``x``."""

        # TODO: update when integration operator is in place: issue #440
        pointwise_norm = self.pointwise_norm(x)
        return pointwise_norm.inner(pointwise_norm.space.one())