@ ProgramNode (location: (1,0)-(1,4))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,4))
    └── body: (length: 1)
        └── @ CallNode (location: (1,0)-(1,4))
            ├── flags: ignore_visibility
            ├── receiver: ∅
            ├── call_operator_loc: ∅
            ├── name: :a
            ├── message_loc: (1,0)-(1,1) = "a"
            ├── opening_loc: ∅
            ├── arguments:
            │   @ ArgumentsNode (location: (1,2)-(1,4))
            │   ├── flags: ∅
            │   └── arguments: (length: 1)
            │       └── @ InstanceVariableReadNode (location: (1,2)-(1,4))
            │           └── name: :@b
            ├── closing_loc: ∅
            └── block: ∅