0(1) element: null
1(2) element: PyMatchStatement
2(3) refutable pattern: x | [x]
3(4,7) WRITE ACCESS: x
4(5,11) refutable pattern: [x]
5(6,11) WRITE ACCESS: x
6(7) matched pattern: [x]
7(8) matched pattern: x | [x]
8(9) element: PyStatementList
9(10) element: PyExpressionStatement
10(12) READ ACCESS: y
11(12) ASSERTTYPE ACCESS: null
12(13) element: PyExpressionStatement
13(14) READ ACCESS: z
14() element: null