def resolver(<EOL>expr, safe=DEFAULT_SAFE, tostr=DEFAULT_TOSTR, scope=DEFAULT_SCOPE,<EOL>besteffort=DEFAULT_BESTEFFORT<EOL>):
raise NotImplementedError()<EOL>
Resolve input expression.

    This function is given such as template resolution function. For wrapping
    test for example.

    :param str expr: configuration expression to resolve.
    :param bool safe: if True (default), run safely execution context.
    :param bool tostr: format the result.
    :param dict scope: execution scope (contains references to expression
        objects).
    :param bool besteffort: if True (default), try to resolve unknown variable
        name with execution runtime.
    :return: resolved expression.