Page "Zero-based numbering" Paragraph 13

Note, however, that a language wishing to index arrays from 1 could simply adopt the convention that every " array address " is represented by a ′ = a – s ; that is, rather than using the address of the first array element, such a language would use the address of an imaginary element located immediately before the first actual element.

The indexing expression for a 1-based index would be the following: