0(1) element: null
1(2) element: PyMatchStatement
2(3,11) refutable pattern: 1
3(4) matched pattern: 1
4(5) element: PyMatchStatement
5(6,9) refutable pattern: 11
6(7) matched pattern: 11
7(8) element: PyExpressionStatement
8(9) READ ACCESS: y11
9(10) element: PyExpressionStatement
10(15) READ ACCESS: y1
11(12,15) refutable pattern: 2
12(13) matched pattern: 2
13(14) element: PyExpressionStatement
14(15) READ ACCESS: y2
15(16) element: PyExpressionStatement
16(17) READ ACCESS: z
17() element: null