Fluture#fork.js

  function Future$fork(rej, res){
    check$fork(this, rej, res);
    let immediate = false;
    let clear = function Future$fork$immediateClear() {
      immediate = true
    };
    const _this = this;
    function maybeClear() {
      if (_this._b && clear) {
        clear()
        clear = null
      }
    }
    clear = fork(this, function Future$fork$rej(x){
      maybeClear();
      rej(x);
    }, function Future$fork$res(x){
      maybeClear();
      res(x);
    });
    if (immediate) maybeClear();
    return clear;
  }