“…Knuth characterised the set of permutations that can be sorted by a single pass through an infinite stack as the set of permutations that avoid 231 [11]. Since then many variants of the problem have been studied, for example [1,2,3,4,5,6,7,8,9,13,14,15,16,17,18]. The set of permutations sortable by a stack of depth 2 and an infinite stack in series has a basis of 20 permutations [7], while for two infinite stacks in series there is no finite basis [12].…”