I agree that there is some confusion between a session and a singaround. However, it really P***s me off when I have gone to the trouble to learn a song or piece of music and find that someone who knows a slightly different version insists on joining in and then persists with their version. My favourite trick, especially in a room full of melodeons, is to put a capo on the banjo and play a well known tune in B flat. Watching their faces is worth the effort. I would add that I have been threatened on one occasion by a melodeon player when I did this. He is now dead( dont read too much into that)