@ ProgramNode (location: (1,0)-(4,8))
├── locals: []
└── statements:
    @ StatementsNode (location: (1,0)-(4,8))
    └── body: (length: 1)
        └── @ CallNode (location: (1,0)-(4,8))
            ├── flags: ignore_visibility
            ├── receiver: ∅
            ├── call_operator_loc: ∅
            ├── name: :p
            ├── message_loc: (1,0)-(1,1) = "p"
            ├── opening_loc: ∅
            ├── arguments:
            │   @ ArgumentsNode (location: (1,2)-(4,8))
            │   ├── flags: ∅
            │   └── arguments: (length: 1)
            │       └── @ CallNode (location: (1,2)-(4,8))
            │           ├── flags: ∅
            │           ├── receiver:
            │           │   @ CallNode (location: (1,2)-(4,4))
            │           │   ├── flags: ∅
            │           │   ├── receiver:
            │           │   │   @ StringNode (location: (1,2)-(1,8))
            │           │   │   ├── flags: ∅
            │           │   │   ├── opening_loc: (1,2)-(1,8) = "<<-END"
            │           │   │   ├── content_loc: (2,0)-(3,0) = "  a\n"
            │           │   │   ├── closing_loc: (3,0)-(4,0) = "  END\n"
            │           │   │   └── unescaped: "  a\n"
            │           │   ├── call_operator_loc: ∅
            │           │   ├── name: :+
            │           │   ├── message_loc: (1,8)-(1,9) = "+"
            │           │   ├── opening_loc: ∅
            │           │   ├── arguments:
            │           │   │   @ ArgumentsNode (location: (1,9)-(4,4))
            │           │   │   ├── flags: ∅
            │           │   │   └── arguments: (length: 1)
            │           │   │       └── @ InterpolatedStringNode (location: (1,9)-(4,4))
            │           │   │           ├── flags: ∅
            │           │   │           ├── opening_loc: (1,9)-(1,10) = "'"
            │           │   │           ├── parts: (length: 2)
            │           │   │           │   ├── @ StringNode (location: (1,10)-(2,0))
            │           │   │           │   │   ├── flags: frozen
            │           │   │           │   │   ├── opening_loc: ∅
            │           │   │           │   │   ├── content_loc: (1,10)-(2,0) = "b\n"
            │           │   │           │   │   ├── closing_loc: ∅
            │           │   │           │   │   └── unescaped: "b\n"
            │           │   │           │   └── @ StringNode (location: (4,0)-(4,3))
            │           │   │           │       ├── flags: frozen
            │           │   │           │       ├── opening_loc: ∅
            │           │   │           │       ├── content_loc: (4,0)-(4,3) = "  c"
            │           │   │           │       ├── closing_loc: ∅
            │           │   │           │       └── unescaped: "  c"
            │           │   │           └── closing_loc: (4,3)-(4,4) = "'"
            │           │   ├── closing_loc: ∅
            │           │   └── block: ∅
            │           ├── call_operator_loc: ∅
            │           ├── name: :+
            │           ├── message_loc: (4,4)-(4,5) = "+"
            │           ├── opening_loc: ∅
            │           ├── arguments:
            │           │   @ ArgumentsNode (location: (4,5)-(4,8))
            │           │   ├── flags: ∅
            │           │   └── arguments: (length: 1)
            │           │       └── @ StringNode (location: (4,5)-(4,8))
            │           │           ├── flags: ∅
            │           │           ├── opening_loc: (4,5)-(4,6) = "'"
            │           │           ├── content_loc: (4,6)-(4,7) = "d"
            │           │           ├── closing_loc: (4,7)-(4,8) = "'"
            │           │           └── unescaped: "d"
            │           ├── closing_loc: ∅
            │           └── block: ∅
            ├── closing_loc: ∅
            └── block: ∅