func (s *segment) shiftBy(v vector.V2) *segment {
dprintf("shifting by %v", v)
s.from = Vertex2(vector.V2(s.from).Add(v))
s.to = Vertex2(vector.V2(s.to).Add(v))
s.line = nil
return s
}
func (v1 Vertex2) distFrom(v2 Vertex2) float64 {
v := vector.V2(v2).Sub(vector.V2(v1))
return v.Length()
}