docs: def iter_variants(self):
          """Iterate over marker information."""
          if not self.has_index:
              raise NotImplementedError("Not implemented when IMPUTE2 file is "
                                        "not indexed (see genipe)")
  
          for name, row in self._impute2_index.iterrows():
              # Seeking to the right place in the file
              f = self._impute2_file
              f.seek(int(row.seek))
              chrom, name, pos, a1, a2 = f.read(1024).split(" ")[:5]
              pos = int(pos)
  
              yield Variant(name, CHROM_STR_ENCODE.get(chrom, chrom), pos,
                            [a1, a2])