//	r >  0: p is after q
//
// If p and q are in different files, p is before q if the filename
// of p sorts lexicographically before the filename of q.
func (p Pos) Cmp(q Pos) int {
	pname := p.RelFilename()
	qname := q.RelFilename()
	switch {
	case pname < qname:
		return -1
	case pname > qname:
		return +1
	}

	pline := p.Line()
	qline := q.Line()
	switch {
	case pline < qline:
		return -1