dimpas - cross ratio in GAP...

spent most of the day teaching a MSc student (not mine, but of our HOD, who is o/seas) a bit of group theory and projective geometry
(in particular, cross ratio of quadriples of points on a projective line)
It turns out it's quite useful to construct certain codes over F₃
Dug up for him two wonderful sets of lecture notes by Peter Cameron: Classical Groups and Projective and Polar Spaces

Finally, I clearly impressed him implementing in 15 minutes in GAP the construction we came up with, and testing that it gives what it should. (But OK, I've been programming in GAP since 1993 :))