MUL means simple multiplication of numbers. It is a techique in which you pass a one value as an argument in a function and that function returns another function to which you pass the second value and the process go on. For example: x*y*z can be representing as:

function mul (x) {
return function (y) { // anonymous function
return function (z) { // anonymous function
return x * y * z; };
};
}