type t = {dx:float; dy:float; dz:float}

val dotproduct : t -> t -> float
val between : Point.t -> Point.t -> t
val opposite : t -> t
val point_along: Point.t -> t -> float -> Point.t
val product: t -> t -> t
val length : t -> float
val length2 : t -> float
val normalize : t -> t
val scale : t -> float -> t
val sub : t -> t -> t