The symbolic package is part of the octave-forge project.
Demo of Anonymous function to symbolic function and back to anonymous function and then the use of the interval pkg.
% this is just a formula to start with
% have fun and change it if you want to.
f=@(x) x.^2 +3*x-1 + 5*x.*sin(x);
% the next 2 line take the Anonymous function into a symbolic formula
% now calculate the derivative of the function
% and convert it back to an Anonymous function
% this uses the interval pkg. to find all the roots between -15 an 10
fzero (f, infsup (-15, 10), df)
ans ⊂ 4×1 interval vector
[-5.743488743719015, -5.743488743719013] [-3.0962279604822407, -3.09622796048224] [-0.777688831121563, -0.7776888311215626] [0.22911205809043574, 0.2291120580904359]