docs: def flip_alleles(genotypes):
      """Flip the alleles of an Genotypes instance."""
      warnings.warn("deprecated: use 'Genotypes.flip_coded'", DeprecationWarning)
      genotypes.reference, genotypes.coded = (genotypes.coded,
                                              genotypes.reference)
      genotypes.genotypes = 2 - genotypes.genotypes
      return genotypes