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