by changes at the start of current_buffer that occurred since W's
current matrix was built. Value is null if no such row exists.
- BEG_UNCHANGED us the number of characters unchanged at the start of
+ BEG_UNCHANGED is the number of characters unchanged at the start of
current_buffer. BEG + BEG_UNCHANGED is the buffer position of the
first changed character in current_buffer. Characters at positions <
BEG + BEG_UNCHANGED are at the same buffer positions as they were