solidity/test/libsolidity/syntaxTests/nameAndTypeResolution/255_using_for_function_on_struct.sol

library D { struct s { uint a; } function mul(s storage self, uint x) public returns (uint) { return self.a *= x; } }
contract C {
    using D for D.s;
    D.s x;
    function f(uint a) public returns (uint) {
        return x.mul(a);
    }
}