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