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AST:
 program
  decl_list
  func_list
   function
    type
     integer
    id
    param_list
     param
      id
      type
       integer
     param
      id
      type
       integer
    decl_list
     var_decl
      type
       integer
    block
     assign
      id
      plus
       id
       id
    return_stm
     id
  main
   decl_list
    var_decl
     type
      integer
    var_decl
     type
      integer
    var_decl
     type
      integer
    const_decl
     type
      integer
     value
      number
   block
    assign
     id
     number
    assign
     id
     id
    assign
     id
     func_call
      id
      arg_list
       id
       id