docs: public T get(String variable, String frame, int iteration, FrameIter parentFrameIter, boolean enforceExistence) {
          //TODO eventually we'll cache and reuse VarId objects here to avoid garbage generation on lookup etc
          VarId varId = newVarId(variable, frame, iteration, parentFrameIter);
          T out = nodeOutputs.get(varId);
          if(enforceExistence) {
              Preconditions.checkNotNull(out, "No output found for variable %s (frame %s, iteration %s)", variable, frame, iteration);
          }
          return out;
      }