afni.nimh.nih.gov

Yes. To be accurate,

-gltsym 'SYM: +NF -F' \
-glt_label 1 NF_vs_F \

The label can be almost anything that's useful to help you remember. It could be 'NF-F' or 'NFminusF', if you like one of those more.

The SYMbolic expression needs to be carefully formatted. In particular, you need the space between the '+NF' and the '-F'.