@ ProgramNode (location: (1,0)-(1,7))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,7))
    └── body: (length: 1)
        └── @ CallNode (location: (1,0)-(1,7))
            ├── flags: attribute_write
            ├── receiver:
            │   @ CallNode (location: (1,0)-(1,1))
            │   ├── flags: variable_call
            │   ├── receiver: ∅
            │   ├── call_operator_loc: ∅
            │   ├── name: :a
            │   ├── message_loc: (1,0)-(1,1) = "a"
            │   ├── opening_loc: ∅
            │   ├── arguments: ∅
            │   ├── closing_loc: ∅
            │   └── block: ∅
            ├── call_operator_loc: ∅
            ├── name: :[]=
            ├── message_loc: (1,1)-(1,3) = "[]"
            ├── opening_loc: (1,1)-(1,2) = "["
            ├── arguments:
            │   @ ArgumentsNode (location: (1,6)-(1,7))
            │   ├── flags: ∅
            │   └── arguments: (length: 1)
            │       └── @ CallNode (location: (1,6)-(1,7))
            │           ├── flags: variable_call
            │           ├── receiver: ∅
            │           ├── call_operator_loc: ∅
            │           ├── name: :b
            │           ├── message_loc: (1,6)-(1,7) = "b"
            │           ├── opening_loc: ∅
            │           ├── arguments: ∅
            │           ├── closing_loc: ∅
            │           └── block: ∅
            ├── closing_loc: (1,2)-(1,3) = "]"
            └── block: ∅
