Function drive 

pub fn drive(m: &mut Machine, floor: usize, r: i32) -> i32

The backtracking driver: pops choice points down to floor, rewinding and retrying, and unwinds errors to the nearest catch frame. Shared by the top level and findall/3’s bounded sub-search. Returns 1 if a continuation stopped enumeration; otherwise 0 (CP stack drained to floor, or an uncaught error left in m.error for the next layer out).