[BioPython] Superimposing CA atoms of a chain

[Ramon Crehuet]

       I'd like to superimpose two chains (all atoms from all residues) but 
    calculating the RMS only from CA atoms. That is, I'd like to calculate 
    the transformation matrix for the CA atoms and apply it to all atoms. (A 
    common operation, I guess...)
        Can I do that with the PDB.superimpose module? Otherwise, if I need 
    to use the SVDSuperimpose, can I manipule atom instances or it only 
    works with numeric arrays?
    Thanks,
    Ramon