lu'erma fu'ivla

x1 (letteral) is a letter that occurs in word x2 (text); x2 contains an occurrence of x1 as a digit/term in a sequence

See also: lu'erfu (for words over an alphabet), lerfu (for letters/digits/terms/elements/members of an alphabet; for completely formal usage (without any meaning), use: se zilterle'u)

In notes:

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)