### 2.28.7 Common denominator : `comDenom`

`comDenom` takes as argument a sum of rational fractions.

`comDenom` rewrite the sum as a unique rational fraction.
The denominator of this rational fraction is the common denominator of the
rational fractions given as argument.

Input :

`comDenom(x-1/(x-1)-1/(x``^`

`2-1))`

Output :

`(x``^`

`3+-2*x-2)/(x``^`

`2-1)`