@ ProgramNode (location: (0...24))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...24))
    └── body: (length: 2)
        ├── @ AliasGlobalVariableNode (location: (0...11))
        │   ├── new_name:
        │   │   @ GlobalVariableReadNode (location: (6...8))
        │   │   └── name: :$a
        │   ├── old_name:
        │   │   @ BackReferenceReadNode (location: (9...11))
        │   └── keyword_loc: (0...5) = "alias"
        └── @ AliasGlobalVariableNode (location: (13...24))
            ├── new_name:
            │   @ GlobalVariableReadNode (location: (19...21))
            │   └── name: :$a
            ├── old_name:
            │   @ GlobalVariableReadNode (location: (22...24))
            │   └── name: :$b
            └── keyword_loc: (13...18) = "alias"
