Collaborative collection of useful userRPL snippets

[Paul Dale]

I just had a quick look and noticed that the angle between vectors snippet is using the arccosine of the dot product. This has some numeric issues. Better is this formula:

\[angle\_between(\,\vec v_1, \vec v_2) =
\begin{cases}
\pi - 2 \, arcsin \frac{\mathopen|-\vec v_2 - \vec v_1\mathclose|}{2}, &\quad \vec v_1 \boldsymbol{\cdot} \vec v_2 < 0 \\
2 \, arcsin \frac{\mathopen|\,\vec v_2 - \vec v_1\mathclose|}{2}, &\quad\text{otherwise.} \\
\end{cases}
\]