@ ProgramNode (location: (0...19))
├── locals: []
└── statements:
    @ StatementsNode (location: (0...19))
    └── body: (length: 1)
        └── @ NextNode (location: (0...19))
            ├── arguments:
            │   @ ArgumentsNode (location: (5...19))
            │   └── arguments: (length: 1)
            │       └── @ CallNode (location: (5...19))
            │           ├── receiver: ∅
            │           ├── call_operator_loc: ∅
            │           ├── message_loc: (5...8) = "fun"
            │           ├── opening_loc: ∅
            │           ├── arguments:
            │           │   @ ArgumentsNode (location: (9...12))
            │           │   └── arguments: (length: 1)
            │           │       └── @ CallNode (location: (9...12))
            │           │           ├── receiver: ∅
            │           │           ├── call_operator_loc: ∅
            │           │           ├── message_loc: (9...12) = "foo"
            │           │           ├── opening_loc: ∅
            │           │           ├── arguments: ∅
            │           │           ├── closing_loc: ∅
            │           │           ├── block: ∅
            │           │           ├── flags: variable_call
            │           │           └── name: "foo"
            │           ├── closing_loc: ∅
            │           ├── block:
            │           │   @ BlockNode (location: (13...19))
            │           │   ├── locals: []
            │           │   ├── parameters: ∅
            │           │   ├── body: ∅
            │           │   ├── opening_loc: (13...15) = "do"
            │           │   └── closing_loc: (16...19) = "end"
            │           ├── flags: ∅
            │           └── name: "fun"
            └── keyword_loc: (0...4) = "next"
