code: function lazyConsForce() {
    /* jshint validthis:true */
    var val = this.tailFn();
    /* eslint-disable no-use-before-define */
    this.tailValue = Array.isArray(val) ? fromArray(val) : val;
    /* eslint-enable no-use-before-define */
  
    delete this.tail;
    delete this.force;
  
    return this;
  }
docs: But when cons is created, it's overloaded with lazy ones Force tail to whnf.