A rewrite copied a boxed value's bytes, pointers and all, but wrote the
arrays and strings those pointers reach only when a field also reached
them. Rewritten documents lost that data and the boxed pointers aimed
at whatever moved into its place; the mod merge rewrites real documents
this way. Read documents now keep their EBXD bytes, and the writer
carries every array, string, instance and boxed value a boxed pointer
reaches, repointed. Found by a round-trip check on bam's level root (one string and
about 1,120 bytes of arrays lost).
Dave Johnson
f22adf2348