+.B pool_free_old()
+takes a boundary
+.I addr
+value that was returned by
+.B pool_boundary()
+and frees up any extents in the
+.I pool
+that have data allocated from that point backward in time.
+NOTE: you must NOT mix calls to both
+.B pool_free
+and
+.B pool_free_old
+on the same pool!
+.P
+.B pool_boundary()
+asks for a boundary value that can be sent to
+.B pool_free_old()
+at a later time to free up all memory allocated prior to a particular
+moment in time.
+If the extent that holds the boundary point has allocations from after the
+boundary point, it will not be freed until a future
+.B pool_free_old()
+call encompasses the entirety of the extent's data.
+If
+.I len
+is non-zero, the call will also check if the active extent has at least
+that much free memory available in it, and if not, it will mark the
+extent as inactive, forcing a new extent to be used for future allocations.
+(You can specify -1 for
+.I len
+if you want to force a new extent to start.)
+.P