When I run 4channels, I get:

4channels -N A.NEF
4channels - LibRaw 0.21.1-Release sample. 1182 cameras supported
Usage: 4channels [-s N] [-g] [-A] [-B] [-N] raw-files....
-s N - select Nth image in file (default=0)
-g - use gamma correction with gamma 2.2 (not precise,use for visual inspection only)
-A - autoscaling (by integer factor)
-B - no black subtraction

What does -N do? Since it is described in the Usage line I expected it to work and have an explanation like -B and the others. What gives?

Cheers,

Peter