However , some other permutation classes have bases with more than one pattern or even with infinitely many patterns .
Thus , for instance , the stack - sortable permutations form a principal permutation class , defined by the forbidden pattern 231 .
Some other permutation classes have bases with more than one pattern or even with infinitely many patterns .
Thus , for instance , the stack - sortable permutations form a principal permutation class , defined by the forbidden pattern 231 .