1. function spawn(genF) {
    2.   return new Promise(function(resolve, reject) {
    3.     const gen = genF();
    4.     function step(nextF) {
    5.       let next;
    7.       } catch(e) {
    8.         return reject(e);
    9.       }
    10.       if(next.done) {
    11.         return resolve(next.value);
    12.       }
    13.         step(function() { return gen.next(v); });
    14.       }, function(e) {
    15.         step(function() { return gen.throw(e); });
    16.       });
    17.     }
    18.     step(function() { return gen.next(undefined); });