tail -n: 0 versus +0 versus -0

[mknos]

• I wrote a test harness for tail-zero-lines...
  for opt in '-0' '+0' '-n 0' '-n -0' '-n +0' ; do echo "tail $opt"; perl tail $opt in; done;
• Some cases result in the error: The number cannot be null
• "tail -n 0" is accepted but "tail -n +0" and "tail -n -0" are not
• When debugging I found that check_number() is not called for "tail -n 0" (but it should be)
• check_number() was only called if $opt_b/$opt_c/$opt_n were true
• NB: GNU tail allows -n 0, but raising error in this version is consistent internally, and with "head -n 0"