mekso binary operator: extract substructure/underlying set/endowing operator; the substructure (general sense; includes just operator, order, set, etc.) of X1 (structure; explicitly given by {du'a'e}) which is formed by collecting the ith entries of that {du'a'e}-tuple in order together into their own {du'a'e}-tuple (or by extracting them naked into the ambient environment if X2 is a singleton) for all i in set X2
Usually, a complicated structure will involve the underlying set. i is not a user-submission-accepting terbri; only X1 and X2 are. If X1 is not provided explicitly here by a du'a'e construct, then one must be inferred from context (the most recent definition; the order of things entries submitted to that construct is understood to apply here via direct formal substitution); otherwise, this word is undefined. Even though operators and orders must be submitted to de'a'e via zai'ai-mau'au quotes, they are extracted naked by this word (so that they can be used directly as evaluating operators in numerical expressions; thus, they must be requoted if that is desired/appropriate). Counting starts at 1. Thus, X2 being exactly the singleton of 1 will output the underlying set of X1.