In the last week I didn’t create any PR which is worth to mention so in this
report I’d like to introduce the concept which we’re going to add `vector`

module.

Currently, in SymPy we have transformation equations but they’re responsible
only for rotation and translation. We have already implemented the transformation
equations for changing type of coordinate system so we need to somehow deals with
composition of this two kind of transformation. To do that, firstly we need to
implement a method for obtaining transformation equations from rotation matrix.
Secondly, we need to create generic method to compose two transformation. In
`vector`

module we are in linear regime so from mathematical point of view it is
not complicated. We can do that by SymPy’s `.subs()`

:

```
eq11 = f(x,y,z)
eq12 = g(z,y,z)
eq13 = h(x,y,z) # First set of transformation equations
eq21 = a(x,y,z)
eq22 = b(x,y,z)
eq23 = c(x,y,z) # Second set of transformation equations
eq11.subs(x, eq21)
eq11.subs(y, eq22)
eq11.subs(z, eq23)
```

After setting first set of transformation equations we need to substitute the variables by second set of transformation equations.