/* This structure marks which slots in a buffer have corresponding
default values in buffer_defaults.
- Each such slot has a nonzero value in this structure.
- The value has only one nonzero bit.
+ Each such slot has a value in this structure.
+ The value is a positive Lisp integer that must be smaller than
+ MAX_PER_BUFFER_VARS.
When a buffer has its own local value for a slot,
the entry for that slot (found in the same slot in this structure)