0(1) element: null
1(2) WRITE ACCESS: x
2(3) element: PyMatchStatement
3(4) READ ACCESS: x
4(5,9) refutable pattern: 42
5(6) matched pattern: 42
6(7) ASSERTTYPE ACCESS: x
7(8) element: PyStatementList
8(12) element: PyReturnStatement
9(10) ASSERTTYPE ACCESS: x
10(11) element: PyExpressionStatement
11(12) READ ACCESS: y
12() element: null