@ ProgramNode (location: (1,0)-(1,13))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,13))
    └── body: (length: 1)
        └── @ IfNode (location: (1,0)-(1,13))
            ├── if_keyword_loc: (1,0)-(1,2) = "if"
            ├── predicate:
            │   @ FlipFlopNode (location: (1,3)-(1,8))
            │   ├── flags: ∅
            │   ├── left:
            │   │   @ CallNode (location: (1,3)-(1,6))
            │   │   ├── flags: variable_call, ignore_visibility
            │   │   ├── receiver: ∅
            │   │   ├── call_operator_loc: ∅
            │   │   ├── name: :foo
            │   │   ├── message_loc: (1,3)-(1,6) = "foo"
            │   │   ├── opening_loc: ∅
            │   │   ├── arguments: ∅
            │   │   ├── closing_loc: ∅
            │   │   └── block: ∅
            │   ├── right: ∅
            │   └── operator_loc: (1,6)-(1,8) = ".."
            ├── then_keyword_loc: ∅
            ├── statements: ∅
            ├── consequent: ∅
            └── end_keyword_loc: (1,10)-(1,13) = "end"