projectOnPlane( planeNormal ) {

		_vector$c.copy( this ).projectOnVector( planeNormal );

		return this.sub( _vector$c );

	}
Projects this vector onto a plane by subtracting this
vector projected onto the plane's normal from this vector.

@param {Vector3} planeNormal - The plane normal.
@return {Vector3} A reference to this vector.