ternary mekso operator: retrieves/gets/outputs the X_{2}th entry/term from ordered list X_{1} under indexing rules X_{3}.

If X_{2} is an integer such that, according to X_{3},
X_{2} is inclusively between the least index and the greatest index of X_{1}, then this word outputs the X_{2}th entry. If X_{2} is a set, then the outputs is the ordered list such that each entry thereof is some entry of X_{1} such that its index in X_{1} according to X_{3} belongs to X_{2}; the output list is reindexed (so that the jth entry thereof has index j, where j is a positive integer).