Iterator functions ------------------

Functions that tell you about size. Alas, these aren't so useful because our table is always fixed size.

Check if (alleged) mstate m has expected magic field

This takes the specified elements out of the group. This is "undefining", rather than "clearing". TODO(austern): Make this exception safe: handle exc

We also may want to know how many *used* buckets there are

top_foot_size is padding at the end of a segment, including space that may be needed to place segment records and fenceposts when new noncontiguou