def add_prefix(vrs, prefix):
    """Prepend `prefix` to hidden variables in `vrs`.

    @type vrs: `dict`
    @type prefix: `str`
    """
    d = dict()
    for k, v in vrs.items():
        if k.startswith('_'):
            name = prefix + k
            assert name not in d, (name, d)
        else:
            name = k
        d[name] = v
    return d