======= AST Display =========================
Package
  ClassDeclList [1]
  . ClassDecl
  .   "A" classname
  .   FieldDeclList [0]
  .   MethodDeclList [1]
  .   . (public) MethodDecl
  .   .   INT BaseType
  .   .   "f" methodname
  .   .   ParameterDeclList [0]
  .   .   StmtList [1]
  .   .   . AssignStmt
  .   .   .   IdRef
  .   .   .     "x" Identifier
  .   .   .   BinaryExpr
  .   .   .     "-" Operator
  .   .   .       LiteralExpr
  .   .   .         "1" IntLiteral
  .   .   .       BinaryExpr
  .   .   .         "*" Operator
  .   .   .           LiteralExpr
  .   .   .             "2" IntLiteral
  .   .   .           UnaryExpr
  .   .   .             "-" Operator
  .   .   .               LiteralExpr
  .   .   .                 "3" IntLiteral
=============================================