public static double solve(Expr expr, string varName)
{
Expr v = Expr.Variable(varName);
expr = expr.RationalSimplify(v).Numerator().Expand();
Expr[] coeff = expr.Coefficients(v);
Console.WriteLine(coeff.Length);
switch (coeff.Length)
{
case 1: return(0.0); //Expr.Zero.Equals(coeff[0]) ? v : Expr.Undefined;
case 2: return((-coeff[0] / coeff[1]).Expand().RationalSimplify(v).RealNumberValue);
default: return(0.0); //Expr.Undefined.RealNumberValue;
}
}