Marked_a__taila__zerosa__tailMarked_mark0tailzerosmarkMarked_a__zerosconsX2X1XSXa__zeroscons(0,zeros)a__tail(cons(X,XS))mark(XS)mark(zeros)a__zerosmark(tail(X))a__tail(mark(X))mark(cons(X1,X2))cons(mark(X1),X2)mark(0)0a__zeroszerosa__tail(X)tail(X)a__zeroscons(0,zeros)a__tail(cons(X,XS))mark(XS)mark(zeros)a__zerosmark(tail(X))a__tail(mark(X))mark(cons(X1,X2))cons(mark(X1),X2)mark(0)0a__zeroszerosa__tail(X)tail(X)Marked_a__tail(cons(X,XS))Marked_mark(XS)Marked_mark(zeros)Marked_a__zerosMarked_mark(tail(X))Marked_a__tail(mark(X))Marked_mark(tail(X))Marked_mark(X)Marked_mark(cons(X1,X2))Marked_mark(X1)a__zeroscons(0,zeros)a__tail(cons(X,XS))mark(XS)mark(zeros)a__zerosmark(tail(X))a__tail(mark(X))mark(cons(X1,X2))cons(mark(X1),X2)mark(0)0a__zeroszerosa__tail(X)tail(X)Marked_a__tail(cons(X,XS))Marked_mark(XS)Marked_mark(tail(X))Marked_a__tail(mark(X))Marked_mark(tail(X))Marked_mark(X)Marked_mark(cons(X1,X2))Marked_mark(X1)cons1120mark220zeros0tail32000Marked_mark130a__tail320a__zeros20Marked_a__tail20