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ILL 1990 Problem 40

    Given three letters X,Y,ZX, Y, Z , we can construct letter sequences arbitrarily, such as XZ,ZZYXYY,XXYZXXZ, ZZYXYY, XXYZX , etc. For any given sequence, we can perform following operations: T1T_1 : If the right-most letter is YY , then we can add YZYZ after it, for example, T1(XYZXXY)=(XYZXXYYZ).T_1(XYZXXY) = (XYZXXYYZ). T2T_2 : If The sequence contains YYYYYY , we can replace them by ZZ , for example, T2(XXYYZYYYX)=(XXYYZZX).T_2(XXYYZYYYX) = (XXYYZZX). T3T_3 : We can replace XpXp ( pp is any sub-sequence) by XpXXpX , for example, T3(XXYZ)=(XXYZX).T_3(XXYZ) = (XXYZX). T4T_4 : In a sequence containing one or more ZZ , we can replace the first ZZ by XYXY , for example, T4(XXYYZZX)=(XXYYXYZX).T_4(XXYYZZX) = (XXYYXYZX). T5T_5 : We can replace any of XX,YY,ZZXX, YY, ZZ by XX , for example, T5(ZZYXYY)=(XYXX)T_5(ZZYXYY) = (XYXX) or (XYXYY)(XYXYY) or (ZZYXX).(ZZYXX). Using above operations, can we get XYZZXYZZ from XYZ ?XYZ \ ?

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