def format_resid(pdb_resid):
    """
    Convert a PDB.Chain.Residue id to an MC-Annotate formatted
    residue identifier.
    """
    if pdb_resid[2] == ' ':
        return str(pdb_resid[1])
    else:
        return str(pdb_resid[1]) + '.' + pdb_resid[2]
