0(1) element: null
1(2) element: PyMatchStatement
2(3) refutable pattern: x | [x]
3(7) WRITE ACCESS: x
4(5,10) refutable pattern: [x]
5(6) WRITE ACCESS: x
6(7) matched pattern: [x]
7(8) matched pattern: x | [x]
8(9) element: PyExpressionStatement
9(10) READ ACCESS: y
10(11) element: PyExpressionStatement
11(12) READ ACCESS: z
12() element: null