def CA_TRACE(residue):
    """
    Reduces protein residues to the alpha carbon:
    CA trace only.
    """

    cg_residue = deepcopy(residue)

    for atom in cg_residue.child_dict.keys():
        if cg_residue[atom].name != 'CA':
            cg_residue.detach_child(cg_residue[atom].name)
    
    return cg_residue