//Excerpt from visionary.C.

class closest_unoccupied_to {
	const terminal_t::iterator it;
	const terminal_t::value_type bg;   //background character
public:
	closest_unoccupied_to(const terminal_t::iterator& initial_it,
		terminal_t::value_type initial_bg)
		: it(initial_it), _bg(initial_bg) {}

	//Return true if it1 is a closer unoccupied location to it than it2 is.

	bool operator()(const terminal_t::iterator& it1,
	                const terminal_t::iterator& it2) const {

		//If it1 is already occupied, return false.
		if (*it1 != bg) {
			return false;
		}

		/Arrive here is it1 is unoccupied.
		//If it2 is already occupied, return true.
		if (*it2 != bg) {
			return true;
		}

		//Arrive here if it1 and it2 are both unoccupied.
		return distance(it, it1) < distance(it, it2);
	}
};