code: function getSubflow(_, ctx) {
    var spec = _.$subflow;
    return function(dataflow, key, parent) {
      var subctx = parseDataflow(spec, ctx.fork()),
          op = subctx.get(spec.operators[0].id),
          p = subctx.signals.parent;
      if (p) p.set(parent);
      return op;
    };
  }
docs: Resolve a recursive subflow specification.