======= AST Display =========================
Package
  ClassDeclList [2]
  . ClassDecl
  .   "MainClass" classname
  .   FieldDeclList [0]
  .   MethodDeclList [1]
  .   . (public static) MethodDecl
  .   .   VOID BaseType
  .   .   "main" methodname
  .   .   ParameterDeclList [1]
  .   .   . ParameterDecl
  .   .   .   ArrayType
  .   .   .     ClassType
  .   .   .       "String" classname
  .   .   .   "args"parametername 
  .   .   StmtList [0]
  . ClassDecl
  .   "OfItsOwn" classname
  .   FieldDeclList [1]
  .   . (public) FieldDecl
  .   .   INT BaseType
  .   .   "A_01" fieldname
  .   MethodDeclList [0]
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