+/* Return the index value of the per-buffer variable at offset OFFSET
+ in the buffer structure.
+
+ If the slot OFFSET has a corresponding default value in
+ buffer_defaults, the index value is positive and has only one
+ nonzero bit. When a buffer has its own local value for a slot, the
+ bit for that slot (found in the same slot in this structure) is
+ turned on in the buffer's local_flags array.
+
+ If the index value is -1, even though there may be a
+ DEFVAR_PER_BUFFER for the slot, there is no default value for it;
+ and the corresponding slot in buffer_defaults is not used.
+
+ If the index value is -2, then there is no DEFVAR_PER_BUFFER for
+ the slot, but there is a default value which is copied into each
+ new buffer.
+
+ If a slot in this structure corresponding to a DEFVAR_PER_BUFFER is
+ zero, that is a bug */
+