A structure consisting of an ordered set of sorted lists such that the head and tail entries of later lists nest within earlier ones. For example, an encroaching list
set for 
is given by 
.
Encroaching list sets can be computed using EncroachingListSet[l]
in the Wolfram Language package Combinatorica`
.
It is conjectured that the number of encroaching lists associated with a random permutation of size 
is 
for sufficiently large 
(Skiena 1988; Skiena 1990, p. 78).
