The Lucid direct addressing operator, @, is used to index a particular subset of a stream, along a specified dimension. While it is available within a Q function in a limited form it is not directly supported at the dataflow level, except in special cases such as resample.
A direct addressing primitive in the dataflow graph level might to be needed, but an unbounded version is impractical with a demand driven evaluation model. Instead a relatively general, yet bounded version may be adopted.