mekso string operator (n-ary): formal right-concatenation; X1 + X, where Xi is a string/word/text/character/letteral/lerfu/quoted utterance (quote appropriately iff necessary; preserve and be careful about the use-vs.-mention distinction) for all i.
Generally noncommutative. Xi may be referenced by a defined/named string-type object/variable; for example: if s = 'BC', then joihi('A', s) = 'ABC' (and not 'As'). The result is a single string written over the alphabet that is the union of each of the alphabets of the Xi's. joihi('ABC', 'DEF', 'GHI') = 'ABC' + 'DEF' + 'GHI' = 'ABCDEFGHI'. Use SE conversion for left-concatenation, depending on exact presentation of input strings. See "konkatena".