Like for queries, the symbols [ and ] are used to enclose sub-trees. These symbols are not mapped to operator methods, but to special producer methods producer$push, producer$pop

In RGG, the sub trees created in a production statement are connected on the first node to the left with a Branch edge. Thus, N1 [N2 N3] connect the sub-tree produced by N2 N3 to N1 with a branch edge.