docs: function _singlePromise () {
    let  done, cancel;
    const x = new Promise ( (resolve, reject ) => { 
                                                    done   = resolve
                                                    cancel = reject
                                   })
  
     return { 
                 promise    : x
               , done       : done 
               , cancel     : cancel
               , onComplete : _after(x)
             }
     }