SignedAngle.md

Let $N$ to be normal of plane decided by $a$ and $b$, where $N$ is at the side so that it has the minimal angle between the input normal. Let $angle$ be the unsigned angle between $a$ and $b$. Around $N$, following the right hand rule, if $a$ can rotate to $b$ in positive $angle$ angle, then the result is $angle$. Otherwise, the result is $-angle$.