5.2522 The general term of a formal series a, O’ a, O’ O’ a, … I write thus: “[a, x, O’ x].” This bracketed expression is a variable. The first term of the bracketed expression is the beginning of the formal series, the second the form of an arbitrary term x of the series, and the third the form of the term of the series that follows immediately after x.

OK, more definitions.