old_contents: function ValueClosure(varName, expr) {
    this.varName = varName;
    this.expr = expr;
  }
  ValueClosure.prototype = {
    toString: function () {
      return "#<closure>"
    }
  }
  
new_contents: function ValueClosure(varName, expr) {
    this.varName = varName;
    this.expr = expr;
  }
  ValueClosure.prototype = {
    call: function (arg, env) {
      var newEnv = new EnvLocal(env);
      newEnv.add(this.varName, arg);
      return this.expr.eval(newEnv);
    },
    toString: function () {
      return "#<closure>"
    }
  }
  
subject: Add ValueClosure#call