def _convert(self, chain_id, residue_id, from_scheme, to_scheme):
        '''The actual 'private' conversion function.'''
        if from_scheme == 'rosetta':
            atom_id = self.rosetta_to_atom_sequence_maps.get(chain_id, {})[residue_id]
            if to_scheme == 'atom':
                return atom_id
            else:
                return self._convert(chain_id, atom_id, 'atom', to_scheme)
        if from_scheme == 'atom':
            if to_scheme == 'rosetta':
                return self.atom_to_rosetta_sequence_maps.get(chain_id, {})[residue_id]
            else:
                seqres_id = self.atom_to_seqres_sequence_maps.get(chain_id, {})[residue_id]
                if to_scheme == 'seqres':
                    return seqres_id
                return self.convert(chain_id, seqres_id, 'seqres', to_scheme)
        if from_scheme == 'seqres':
            if to_scheme == 'uniparc':
                return self.seqres_to_uniparc_sequence_maps.get(chain_id, {})[residue_id]
            else:
                atom_id = self.seqres_to_atom_sequence_maps.get(chain_id, {})[residue_id]
                if to_scheme == 'atom':
                    return atom_id
                return self.convert(chain_id, atom_id, 'atom', to_scheme)
        if from_scheme == 'uniparc':
            seqres_id = self.uniparc_to_seqres_sequence_maps.get(chain_id, {})[residue_id]
            if to_scheme == 'seqres':
                return seqres_id
            else:
                return self._convert(chain_id, seqres_id, 'seqres', to_scheme)
        raise Exception("We should never reach this line.")