@ ProgramNode (location: (1,0)-(1,19))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,19))
    └── body: (length: 1)
        └── @ CallNode (location: (1,0)-(1,19))
            ├── flags: ignore_visibility
            ├── receiver: ∅
            ├── call_operator_loc: ∅
            ├── name: :a
            ├── message_loc: (1,0)-(1,1) = "a"
            ├── opening_loc: ∅
            ├── arguments:
            │   @ ArgumentsNode (location: (1,2)-(1,19))
            │   ├── flags: ∅
            │   └── arguments: (length: 1)
            │       └── @ ArrayNode (location: (1,2)-(1,19))
            │           ├── flags: ∅
            │           ├── elements: (length: 2)
            │           │   ├── @ NilNode (location: (1,4)-(1,7))
            │           │   └── @ CallNode (location: (1,9)-(1,17))
            │           │       ├── flags: ignore_visibility
            │           │       ├── receiver: ∅
            │           │       ├── call_operator_loc: ∅
            │           │       ├── name: :b
            │           │       ├── message_loc: (1,9)-(1,10) = "b"
            │           │       ├── opening_loc: ∅
            │           │       ├── arguments: ∅
            │           │       ├── closing_loc: ∅
            │           │       └── block:
            │           │           @ BlockNode (location: (1,11)-(1,17))
            │           │           ├── locals: []
            │           │           ├── parameters: ∅
            │           │           ├── body: ∅
            │           │           ├── opening_loc: (1,11)-(1,13) = "do"
            │           │           └── closing_loc: (1,14)-(1,17) = "end"
            │           ├── opening_loc: (1,2)-(1,3) = "["
            │           └── closing_loc: (1,18)-(1,19) = "]"
            ├── closing_loc: ∅
            └── block: ∅