@ ProgramNode (location: (1,0)-(1,7))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,7))
    └── body: (length: 1)
        └── @ InstanceVariableWriteNode (location: (1,0)-(1,7))
            ├── name: :@a
            ├── name_loc: (1,0)-(1,2) = "@a"
            ├── value:
            │   @ IntegerNode (location: (1,5)-(1,7))
            │   ├── flags: decimal
            │   └── value: 42
            └── operator_loc: (1,3)-(1,4) = "="