Animal Trick / Animal Companion Question

I'm currently using some software (PCGen) to create a plethora of characters (I don't have a problem, I swear) for fun. However, I've come into a few issues here and there regarding it, but most of the time it was simply because I didn't read the source materials well enough.

I'm finding myself in the same situation where I'm having trouble finding something, but as compared to before, I'm finding little to no information, so I figured I'd check here before I submit it as a bug to PCGen.

A first level Hunter with a Lion animal companion ends up being able to select 7 tricks to teach his/her animal companion (6 for having a 2 in Int, plus one bonus trick for being an animal companion.) Looking at the list here, I noticed one in particular I wanted to go for; Flank. However, this particular trick isn't listed. In fact, only Air Walk, Attack, Attack 2, Come, Defend, Down, Fetch, Guard, Heel, Perform, Seek, Stay, Swooping Charge, Track, and Work are listed.

I suspect it's one of the following problems:

• Flank has species/stat prerequisites not listed under Handle Animal, and my lion doesn't meet these prerequisites.

• Flank is in a supplementary text I don't currently have loaded.

• Flank requires you to take other tricks first.

If any of these are true, I'd appreciate if anyone here could tell me what prerequisites Flank needs and/or which supplementary book it's listed in. Alternatively, if anyone is familiar with PCGen and can help me with this issue, it'd be greatly appreciated!

Thanks!