ternary mekso operator: retrieves/gets/outputs the X2th entry/term from ordered list X1 under indexing rules X3.
If X2 is an integer such that, according to X3, X2 is inclusively between the least index and the greatest index of X1, then this word outputs the X2th entry. If X2 is a set, then the outputs is the ordered list such that each entry thereof is some entry of X1 such that its index in X1 according to X3 belongs to X2; the output list is reindexed (so that the jth entry thereof has index j, where j is a positive integer).