gensym = function() {
  gensym.count = gensym.count + 1;
  return "_gensm_" + gensym.count;
};
gensym.count = 0;

Bindings = function() { };
Bindings.prototype.unify = function(x, v) {
  if (this.hasOwnProperty(x) && x != v)
    throw "unable to unify";
  this[x] = v;
};

tryCatch = function(t, c) {
  var w = world.sprout();
  try {
    in w { t(); }
  } catch (e) {
    w = null;
    c();
  } finally {
    if (w != null)
      w.commit();
  }
};

father = function(x, y, bs, k) {
  tryCatch(function() {
             bs.unify(x, "abe");
             bs.unify(y, "homer");
             k(bs)
           },
           function() {
             tryCatch(function() {
                        bs.unify(x, "homer");
                        bs.unify(y, "bart");
                        k(bs)
                      },
                      function() {
                        tryCatch(function() {
                                   bs.unify(x, "homer");
                                   bs.unify(y, "lisa");
                                   k(bs)
                                 },
                                 function() { });
                      });
           });
};

// The tryCatch here is not really needed, since the only side
// effect grandfather does on its own is in gensym, which
// we don't care about.
grandfather = function(x, y, bs, k) {
  tryCatch(function() {
             var z = gensym();
             father(x, z, bs, function(bs) { father(z, y, bs, k) })
           },
           function() { });
};