functionSet.js

function emptySet(x) {
    return false;
}

function insert(elem, set) {
     return function(x) {
         if (x === elem) { // insert smarter comparison here if needed
             return true;
         }
         return function() { return contains(x, set); };
     };
}

function contains(elem, set) {
    var retVal = set(elem);

    while(typeof retVal === 'function') {
        retVal = retVal(elem);
    }
    return retVal;
}


var aList = insert(3, insert(2, insert(1, emptySet)));

console.log("FIRST: " + contains(2, aList));
console.log("SECOND: " + contains(4, aList));