Following on from G forces Comments above;   I play regularly in an English Country Dance band, … we are only human and cannot possibly know every tune that's ever been written, and if a caller asks for a tune we don't know, we substitute … there is usually no problem. Callers rarely ask for a specific tune, usually they ask for,say, 32 bar jigs, or 48 bar polka's or something similar, very few dances actually have specific tunes attached to them anyway.