======= AST Display =========================
Package
  ClassDeclList [1]
  . ClassDecl
  .   "A" classname
  .   FieldDeclList [0]
  .   MethodDeclList [1]
  .   . (public) MethodDecl
  .   .   VOID BaseType
  .   .   "f" methodname
  .   .   ParameterDeclList [2]
  .   .   . ParameterDecl
  .   .   .   INT BaseType
  .   .   .   "a"parametername 
  .   .   . ParameterDecl
  .   .   .   INT BaseType
  .   .   .   "b"parametername 
  .   .   StmtList [1]
  .   .   . IfStmt
  .   .   .   BinaryExpr
  .   .   .     "<" Operator
  .   .   .       RefExpr
  .   .   .         IdRef
  .   .   .           "a" Identifier
  .   .   .       RefExpr
  .   .   .         IdRef
  .   .   .           "b" Identifier
  .   .   .   AssignStmt
  .   .   .     IdRef
  .   .   .       "b" Identifier
  .   .   .     RefExpr
  .   .   .       IdRef
  .   .   .         "a" Identifier
=============================================