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Thank you for help. Your answer helped me a lot. There are many problems with this OOP-related-features in Mathematica. And one is solved due to your explanation. Have a Good day.
Thank you for your answer, Mr.Reiss. What you said was very curious and I am now trying to understand that. It is helpful. :) And additionally, however, What I exactly wanna do is deleting just one line of last printed output, not the whole...