Generator for the definition domain of a function

[gchabert]

We need a function

const System& Function::domain()

that generates the set of inequalities corresponding to the definition domain of the function.

[gchabert]

Example of use:

    Function f("x","y","ln(x)+sqrt(x+y)");
    const System& sys=f.def_domain();
    cout << sys << endl;

Prints:

 x>=0
 (x+y)>=0