cross method
Cross a with this vector if b is provided cross a with b
Implementation
Vec3 cross(Vec3 a, [Vec3? b ]) {
if ( b != null ) return crossVectors( a, b );
double x = this.x, y = this.y, z = this.z;
this.x = y * a.z - z * a.y;
this.y = z * a.x - x * a.z;
this.z = x * a.y - y * a.x;
return this;
}