start -> expr [SimNode]
  expr -> expr assign-op (^!)15 expr (>!)14 [SimNode]
    expr -> IDENT [SimNode]
      IDENT
    assign-op -> ASSIGN [SimNode]
      ASSIGN
    expr -> expr shift-op (^!)5 expr (>!)4 [SimNode]
      expr -> NUMBER [SimNode]
        NUMBER
      shift-op -> LSHIFT [SimNode]
        LSHIFT
      expr -> expr add-op (^!)3 expr >>!! [SimNode]
        expr -> NUMBER [SimNode]
          NUMBER
        add-op -> PLUS [SimNode]
          PLUS
        expr -> expr mul-op ^! expr [SimNode]
          expr -> NUMBER [SimNode]
            NUMBER
          mul-op -> TIMES [SimNode]
            TIMES
          expr -> NUMBER [SimNode]
            NUMBER