contract test {
    mapping(uint8 => uint8) m1;
    mapping(uint8 => uint8) m2;
    function f() public returns (uint8, uint8, uint8, uint8) {
        mapping(uint8 => uint8) storage m = m1;
        m[1] = 42;

        m = m2;
        m[2] = 21;

        return (m1[1], m1[2], m2[1], m2[2]);
    }
}
// ----
// f() -> 42, 0, 0, 21