rnrs mutable-pairs

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.