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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_real(self, x_real, out_real):
 """Real-space forward projection for the current set-up. This method also sets ``self._astra_projector`` for ``impl='astra_cuda'`` and enabled cache. """

        if self.impl.startswith('astra'):
            backend, data_impl = self.impl.split('_')

            if data_impl == 'cpu':
                return astra_cpu_forward_projector(
                    x_real, self.geometry, self.range.real_space, out_real)

            elif data_impl == 'cuda':
                if self._astra_wrapper is None:
                    astra_wrapper = AstraCudaProjectorImpl(
                        self.geometry, self.domain.real_space,
                        self.range.real_space)
                    if self.use_cache:
                        self._astra_wrapper = astra_wrapper
                else:
                    astra_wrapper = self._astra_wrapper

                return astra_wrapper.call_forward(x_real, out_real)
            else:
                # Should never happen
                raise RuntimeError('bad `impl` {!r}'.format(self.impl))
        elif self.impl == 'skimage':
            return skimage_radon_forward(x_real, self.geometry,
                                         self.range.real_space, out_real)
        else:
            # Should never happen
            raise RuntimeError('bad `impl` {!r}'.format(self.impl))