compUnit
|-- funcDef
|   |-- funcType
|   |   `-- Int: int
|   |-- Ident: main
|   |-- L_PAREN: (
|   |-- R_PAREN: )
|   `-- block
|       |-- L_BRACE: {
|       |-- blockItem
|       |   `-- stmt
|       |       |-- Return: return
|       |       |-- exp
|       |       |   `-- addExp
|       |       |       `-- mulExp
|       |       |           `-- unaryExp
|       |       |               `-- primaryExp
|       |       |                   `-- number
|       |       |                       `-- IntConst: 3
|       |       `-- Semi: ;
|       `-- R_BRACE: }
`-- EOF: <EOF>