@ ProgramNode (location: (1,0)-(3,3))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(3,3))
    └── body: (length: 1)
        └── @ CallNode (location: (1,0)-(3,3))
            ├── receiver: ∅
            ├── call_operator_loc: ∅
            ├── message_loc: (1,0)-(1,1) = "f"
            ├── opening_loc: (1,1)-(1,2) = "("
            ├── arguments:
            │   @ ArgumentsNode (location: (1,2)-(1,3))
            │   ├── arguments: (length: 1)
            │   │   └── @ CallNode (location: (1,2)-(1,3))
            │   │       ├── receiver: ∅
            │   │       ├── call_operator_loc: ∅
            │   │       ├── message_loc: (1,2)-(1,3) = "a"
            │   │       ├── opening_loc: ∅
            │   │       ├── arguments: ∅
            │   │       ├── closing_loc: ∅
            │   │       ├── block: ∅
            │   │       ├── flags: variable_call
            │   │       └── name: :a
            │   └── flags: ∅
            ├── closing_loc: (1,3)-(1,4) = ")"
            ├── block:
            │   @ BlockNode (location: (1,5)-(3,3))
            │   ├── locals: [:x, :y]
            │   ├── parameters:
            │   │   @ BlockParametersNode (location: (1,8)-(1,14))
            │   │   ├── parameters:
            │   │   │   @ ParametersNode (location: (1,9)-(1,13))
            │   │   │   ├── requireds: (length: 2)
            │   │   │   │   ├── @ RequiredParameterNode (location: (1,9)-(1,10))
            │   │   │   │   │   └── name: :x
            │   │   │   │   └── @ RequiredParameterNode (location: (1,12)-(1,13))
            │   │   │   │       └── name: :y
            │   │   │   ├── optionals: (length: 0)
            │   │   │   ├── rest: ∅
            │   │   │   ├── posts: (length: 0)
            │   │   │   ├── keywords: (length: 0)
            │   │   │   ├── keyword_rest: ∅
            │   │   │   └── block: ∅
            │   │   ├── locals: (length: 0)
            │   │   ├── opening_loc: (1,8)-(1,9) = "|"
            │   │   └── closing_loc: (1,13)-(1,14) = "|"
            │   ├── body:
            │   │   @ StatementsNode (location: (2,2)-(2,7))
            │   │   └── body: (length: 1)
            │   │       └── @ CallNode (location: (2,2)-(2,7))
            │   │           ├── receiver:
            │   │           │   @ LocalVariableReadNode (location: (2,2)-(2,3))
            │   │           │   ├── name: :x
            │   │           │   └── depth: 0
            │   │           ├── call_operator_loc: ∅
            │   │           ├── message_loc: (2,4)-(2,5) = "+"
            │   │           ├── opening_loc: ∅
            │   │           ├── arguments:
            │   │           │   @ ArgumentsNode (location: (2,6)-(2,7))
            │   │           │   ├── arguments: (length: 1)
            │   │           │   │   └── @ LocalVariableReadNode (location: (2,6)-(2,7))
            │   │           │   │       ├── name: :y
            │   │           │   │       └── depth: 0
            │   │           │   └── flags: ∅
            │   │           ├── closing_loc: ∅
            │   │           ├── block: ∅
            │   │           ├── flags: ∅
            │   │           └── name: :+
            │   ├── opening_loc: (1,5)-(1,7) = "do"
            │   └── closing_loc: (3,0)-(3,3) = "end"
            ├── flags: ∅
            └── name: :f
