Convert to a symbolic polynomial : r2e poly2symb

### 2.24.1 Convert to a symbolic polynomial : `r2e poly2symb`

`r2e` or `poly2symb` takes as argument

- a list of coefficients of a polynomial (by decreasing order) and a symbolic variable name (by default `x`)
- or a sum of monomials `%%%{coeff,[n1,....nk] %%%}` and a vector of symbolic variables `[x1,...,xk]`).

`r2e` or `poly2symb` transforms the argument into a symbolic polynomial.
Example with univariate polynomials, input :

`r2e([1,0,-1],x)`

or :

`r2e([1,0,-1])`

or :

`poly2symb([1,0,-1],x)`

Output :

`x*x-1`

Example with sparse multivariate polynomials, input:

`poly2symb(%%%{1,[2]%%%}+%%%{-1,[0]%%%},[x])`

or :

`r2e(%%%{1,[2]%%%}+%%%{-1,[0]%%%},[x])`

Output :

`x```^2```-1`

Input :

`r2e(%%%{1,[2,0]%%%}+%%%{-1,[1,1]%%%}+%%%{2,[0,1]%%%},[x,y])`

or :

`poly2symb(%%%{1,[2,0]%%%}+%%%{-1,[1,1]%%%}+%%%{2,[0,1]%%%},[x,y])`

Output :

`x```^```2-x*y+2*y`