typedef enum {i,j} index_t;
typedef enum {a,b} item_t;
typedef struct { index_t a, item_t b } struct_t;

place p (0, 6) struct_t: struct_t p: (p.b == is item_t a ? 2 : 1)#p;

trans t
in {
  place p: index_t e: ({e, .n}, {e, +.n}, {+e, .n});
};