The (rnrs mutable-pairs (6)) library provides the set-car!
and set-cdr! procedures, which allow the car and
cdr fields of a pair to be modified.
These procedures are identical to the ones provide by Guile’s core
library. See Pairs, for documentation. All pairs in Guile are
mutable; consequently, these procedures will never throw the
&assertion condition described in the R6RS libraries
specification.