Notation of unit objects

### 5.1.1 Notation of unit objects

A unit object has two parts : a real number and a unit expression (a single unit or multiplicative combinaison of unit). The two parts are linked by the character `_` ("underscore"). For example `2_m` for 2 meters. For composite units, parenthesis must be used, e.g. `1_(m*s)`.
If a prefix is put before the unit then the unit is multiplicated by a power of 10. For example `k` or `K` for kilo (indicate a multiplication by 103), `D` for deca (indicate a multiplication by 10), `d` for deci (indicate a multiplication by 10−1) etc...
Input :

`10.5_m`

Output :

`a unit object of value 10.5 meters`

Input :

`10.5_km`

Output :

`a unit object of value 10.5 kilometers`