This seems to be a proper 2-combinator base of length 6:
? = ~,>+ (B)(A) -- ((AB))((AB)) ! = -< (B)(A) -- B < = ()! [for convenience, U is an abbreviation for (())?!<!] > = ()?U - = >U ~ = >(>)?U<<>?U<< + = ()?<~< , = ~?U<
-PkmnQ (talk) 10:13, 28 April 2025 (UTC)