Extended Abstract: Mutable Objects with Several Implementations

This extended abstract outlines an ACL2 feature, attach-stobj, that first appeared in ACL2 Version 8.6 (October, 2024). This feature supports different executable operations for a given abstract stobj, without requiring recertification of the book that introduces that stobj or theorems about it. The paper provides background as well as a user-level overview and some implementation notes.