to remove stray markers to avoid new objects from getting them, but the
problem doesn't seem to be fixed yet.

Rev: src/array.c:1.66
Rev: src/gc.c:1.74
Rev: src/gc.h:1.37
Rev: src/mapping.c:1.78
Rev: src/multiset.c:1.19
Rev: src/object.c:1.116
Rev: src/object.h:1.44
Rev: src/program.c:1.231
Rev: src/threads.h:1.87