Added in version 184.108.40.206 of package base.
If val is #f, the current mapping for key is reported.
If val is not #f, and no value has been installed for key, then the value is installed and #f is returned. If a value has already been installed, then no new value is installed and the old value is returned. The given val must not refer to garbage-collected memory.
This function is intended for infrequent use with a small number of keys.