x1=l2 is a formal character-set/alphabet containing letter(s)/characters/digits/terms x2=l1; x2 is a letter in formal alphabet x1 and is treated formally (it has no meaning, other than possibly being distinct from other letters in x1)
Used for formal language analysis. See also: lu'erma, lu'erfu, lerfu (for a nonformal, potentially meaningful variant - with reordered terbri)