2 つのベクトルの内積
lhs . rhs を返します。
For normalized vectors Dot returns 1 if they point in exactly the same direction;
-1 if they point in completely opposite directions; and a number in between for other
cases (e.g. Dot returns zero if vectors are perpendicular).
For vectors of arbitrary length the Dot return values are similar: they get larger when the angle between vectors
decreases.