def traverse_using(iterator, obj, visitors):
    for target in iterator:
        meth = visitors.get(target.__visit_name__, None)
        if meth:
            meth(target)
    return obj
visit the given expression structure using the given iterator of
dataset/ETHPy150Open RoseOu/flasky/venv/lib/python2.7/site-packages/sqlalchemy/sql/visitors.py/traverse_using