def where(cond, a, b, use_numexpr=True):
if use_numexpr:<EOL><INDENT>return _where(cond, a, b)<EOL><DEDENT>return _where_standard(cond, a, b)<EOL>
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)