Binary tree in JavaScript

binarytree.js

function cons(x, y) {
  return function(w) { return w(x, y) };
};

function car(z) {
  return z(function(x, y) { return x });
};

function cdr(z) {
  return z(function(x, y) { return y });
};

function make_tree(entry, left, right) {
  return cons(entry, cons(left, cons(right, null)));
}

function entry(tree) {
  return car(tree);
}

function left_branch(tree) {
  return car(cdr(tree));
}

function right_branch(tree) {
  return car(cdr(cdr(tree)));
}

/** assumes binary search tree **/
function element_of_set(value, tree) {
  if (!tree) return false;
  var current = entry(tree);
  if (current == value) return true;
  if (current > value) return element_of_set(value, left_branch(tree));
  return element_of_set(value, right_branch(tree));
}

/** make a simple binary search tree **/
var tree = make_tree(4,
                     make_tree(2,
                               make_tree(1, null, null),
                               make_tree(3, null, null)),
                     make_tree(6,
                               make_tree(5, null, null),
                               make_tree(7, null, null)));

document.writeln(element_of_set(7, tree));
document.writeln(element_of_set(3.5, tree));