code: def explicit_counts_map(self, pixels=None):
          
          # No pixel index, so build one
          if self.hpx._ipix is None:
              if self.data.ndim == 2:
                  summed = self.counts.sum(0)
                  if pixels is None:
                      nz = summed.nonzero()[0]
                  else:
                      nz = pixels
                  data_out = np.vstack(self.data[i].flat[nz]
                                       for i in range(self.data.shape[0]))
              else:
                  if pixels is None:
                      nz = self.data.nonzero()[0]
                  else:
                      nz = pixels
                  data_out = self.data[nz]
              return (nz, data_out)
          else:
              if pixels is None:
                  return (self.hpx._ipix, self.data)
          # FIXME, can we catch this
          raise RuntimeError(
              'HPX.explicit_counts_map called with pixels for a map that already has pixels')
docs: return a counts map with explicit index scheme
  
          Parameters
          ----------
          pixels : `np.ndarray` or None
              If set, grab only those pixels.  
              If none, grab only non-zero pixels