Who decided the bc math library will define sine cosine and arctangent?

If you load the [bc math library](https://www.gnu.org/software/bc/manual/html_node/bc_18.html) you get the trig functions s() and c() and a() which are sine, cosine, and arctangent respectively. Why these three functions? I know why it's those three from the mathematical perspective: it's because those are the three you need to translate directly between Cartesian and polar coordinates. I'm a math teacher, and this is unfortunately the only place I've seen sine/cosine/arctangent established as the set of *primitive* trigonometric functions, so I was hoping someone could tell me *why* in a more historical context. Idk I mostly need ammo when talking with math educators about why it's not a blasphemous idea to introduce arctangent to students before tangent.