During week 7 I continued my work on transformation equations for rotation.
One PR #12960 which obtains transformation
equations from rotation matrix was merged. I have prepared also method for composition of
transformation equation but I didn’t push them. I need to still think how to arrange the whole
structure. My mentor suggest that we can just use transformation equations instead of rotation
matrix, because it’s just general concept. After that we could modify constructor in `CoordSys3D`

class and put them transformation equation instead of rotation matrix. I think it’s good idea but
it introduce lots of work with adaption of method which deals with rotation of coordinate system.

The second topic which I got involved was unevaluated expression. In PR #12987
we have added class `Dot`

and `Cross`

. In this Pr we also add possibility to call ‘dot’ and ‘cross’
products independently from `Vector`

class.

For `directional_derivative`

there are still some work. We need to create class which will
be callable if no scalar or vector is put and otherwise not. The simplest solution is to
create two separate class but I’m trying to omit this because I think that it’s not intuitive for user.