binary mekso operator: Let the inputs X_{1} and X_{2} be sets in the same universal set O; then the result of this operator applied to them is X_{1^c
\cap X}, where for any A \subseteq O, A^{c }= O \setminus A.

This is a dual to xa'ei'o and largely has the same properties; only one of them is needed in any operator system.

- xa'ei'o
*(exp!)* - binary mekso operator: Let the inputs X
_{1}and X_{2}be sets in the same universal set O; then the result of this operator applied to them is X_{1^c \cup X}, where for any A \subseteq O, A^{c }= O \setminus A.