def support_issubset(u, vrs, fol):
    """Return `support(u) <= vrs`.

    Use this function to ensure that only the expected
    variables occur in the expression that a BDD represents.
    This check catches several errors early.

    If `fol` is a `dd.cudd.BDD`, then variable names
    will be bits instead of first-order.

    @param vrs: `set` of variable names as `str`
    """
    support = fol.support(u)
    return support.issubset(vrs)
