x1 (quote) is a text/word/sequence wherein the characters/phonemes are taken over alphabet x2 (set)
For formal language analysis. In mathematical writing, this relationship is sometimes written as "/"; the set of all x1 (where x2 is specified) is x2* (where "*" is the Kleene star); then x1 is an element/member of x2*, and the letters of x1 are elements/members of x2. While each letter in x1 must be an element of x2, there can be elements of x2 that do not appear in x1.