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