evaluate the where condition cond on a and b

        Parameters
        ----------

        cond : a boolean array
        a :    return if cond is True
        b :    return if cond is False
        use_numexpr : whether to try to use numexpr (default True)

def where(cond, a, b, use_numexpr=True):
    """ evaluate the where condition cond on a and b

        Parameters
        ----------

        cond : a boolean array
        a :    return if cond is True
        b :    return if cond is False
        use_numexpr : whether to try to use numexpr (default True)
        """

    if use_numexpr:
        return _where(cond, a, b)
    return _where_standard(cond, a, b)