From Discworld MUD Wiki
I'm pretty sure Riposte has a more complex learning level than just x levels in one skill, as I had 35 tactics and 46 parrying and when increasing parrying to 62 it auto-taught riposte to me.
Old GRiME starts to teach you 16 levels of fighting.defence.parrying for 283481 xp. You realise that you could use your newly improved parry to "riposte" someone's attack. Saving... Saving... You finish learning 16 levels of fighting.defence.parrying from Old GRiME.