function next(err, inst) {
    if (err || inst) { return cb(err, inst); }
    var mech = mechs[idx++];
    if (!mech) { return cb(new Error("Unable to instantiate '" + id + "'")); }
    try {
      var arity = mech.length;
      if (arity == 2) {
        mech(mod, next);
      } else {
        var ins = mech(mod);
        next(null, ins);
      }
    } catch (ex) {
      next(ex);
    }
  }
Instantiate `mod` identified by `id`.

@param {Object} mod
@param {String} id
@param {Function} cb
@api protected