def regularize_truncation(truncation):
    """ regularize_truncation( (code, cutoff) ) -> (N1b,N2b)

    Converts a mnemonic description ("ob",N1b) or ("tb",N2b) to the
    appropriate (N1b,N2b) pair for a two-body interaction file.
    """

    (code, N) = truncation
    if (code == "ob"):
        return (N,2*N)
    elif (code == "tb"):
        return (N,N)