The (rnrs mutable-strings (6)) library provides the
string-set! and string-fill! procedures, which allow the
content of strings to be modified “in-place.”
These procedures are identical to the ones provided by Guile’s core
library. See String Modification, for documentation. All strings in
Guile are mutable; consequently, these procedures will never throw the
&assertion condition described in the R6RS libraries
specification.