Returns a pointer to a Mutable version of this record.
Returns a pointer to a Mutable version of this record.
If the underlying implementation is mutable, this will be returned.
If the underlying implementation is immutable, a mutable copy will be returned.
After mutating the instance returned by this method, the original instance
(on which mutable was called) will be in an undefined state. It may or may
not have been modified, depending on whether it was immutable or not.
This is included as an optimization for when we want access to a Mutable record but don't want to pay the cost of copying every time.