@ ProgramNode (location: (1,0)-(1,32))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(1,32))
    └── body: (length: 1)
        └── @ DefNode (location: (1,0)-(1,32))
            ├── name: :exec
            ├── name_loc: (1,9)-(1,13) = "exec"
            ├── receiver:
            │   @ SelfNode (location: (1,4)-(1,8))
            ├── parameters:
            │   @ ParametersNode (location: (1,14)-(1,17))
            │   ├── requireds: (length: 1)
            │   │   └── @ RequiredParameterNode (location: (1,14)-(1,17))
            │   │       └── name: :cmd
            │   ├── optionals: (length: 0)
            │   ├── rest: ∅
            │   ├── posts: (length: 0)
            │   ├── keywords: (length: 0)
            │   ├── keyword_rest: ∅
            │   └── block: ∅
            ├── body:
            │   @ StatementsNode (location: (1,21)-(1,32))
            │   └── body: (length: 1)
            │       └── @ CallNode (location: (1,21)-(1,32))
            │           ├── flags: ∅
            │           ├── receiver: ∅
            │           ├── call_operator_loc: ∅
            │           ├── name: :system
            │           ├── message_loc: (1,21)-(1,27) = "system"
            │           ├── opening_loc: (1,27)-(1,28) = "("
            │           ├── arguments:
            │           │   @ ArgumentsNode (location: (1,28)-(1,31))
            │           │   ├── flags: ∅
            │           │   └── arguments: (length: 1)
            │           │       └── @ LocalVariableReadNode (location: (1,28)-(1,31))
            │           │           ├── name: :cmd
            │           │           └── depth: 0
            │           ├── closing_loc: (1,31)-(1,32) = ")"
            │           └── block: ∅
            ├── locals: [:cmd]
            ├── locals_body_index: 1
            ├── def_keyword_loc: (1,0)-(1,3) = "def"
            ├── operator_loc: (1,8)-(1,9) = "."
            ├── lparen_loc: (1,13)-(1,14) = "("
            ├── rparen_loc: (1,17)-(1,18) = ")"
            ├── equal_loc: (1,19)-(1,20) = "="
            └── end_keyword_loc: ∅
