non-logical connective/mekso operator - variable arity: X1 \ X2
Ordered: 'x1 kei'i x2' is not generally the same as/equivalent to 'x2 kei'i x1'. If X1 is not explicitly specified, it is taken to be some universal set in the discourse (of which all other mentioned sets are subsets, at the least); in this case, the word operates more as the set (absolute) complement. When X1 is specified, it represents the set relative complement. Somewhat analogous to logical NOT (just as intersection is analogous to logical AND, union is analogous to logical AND-OR). The preferred description/name in English is "set (theoretic) exclusion".