(import (rnrs))                     ;R6RS
(import (rnrs base))                ;R6RS
(import (scheme r5rs))              ;R7RS
(import (scheme base))              ;R7RS

(string=? string1 string2 string3 ...)
(string<? string1 string2 string3 ...)
(string>? string1 string2 string3 ...)
(string<=? string1 string2 string3 ...)
(string>=? string1 string2 string3 ...)

The ordering procedures return #t if their arguments are (respectively): monotonically increasing, monotonically decreasing, monotonically nondecreasing, or monotonically nonincreasing. The behavior of the ordering procedures is different in R6RS and R7RS:

R6RS

These procedures are the lexicographic extensions to strings of the corresponding orderings on characters. For example, string<? is the lexicographic ordering on strings induced by the ordering char<? on characters. If two strings differ in length but are the same up to the length of the shorter string, the shorter string is considered to be lexicographically less than the longer string.

R7RS

The ordering procedures compare strings in an implementation-defined way. They may work the same as in R6RS, or they could use some other ordering.

These predicates are required to be transitive. In all cases, a pair of strings must satisfy exactly one of string<?, string=?, and string>?, and must satisfy string<=? if and only if they do not satisfy string>? and string>=? if and only if they do not satisfy string<?.

(string=? "Straße" "Strasse")  =>  #f
(string<? "z" "ß")             =>  #t
(string<? "z" "zz")            =>  #t
(string<? "z" "Z")             =>  #f

R5RS and earlier reports only provided two-argument variants.

R6RS implementations all work the same.

R7RS implementations may lack support for some characters. The order is also implementation-defined, with lexicographical ordering only being one option.