from an 'equation' tlist, recover the corresponding equation.
EXAMPLE
eq='a*x^2+b*sin(0.5*x)';an_eq=analyse_eq(eq);deriv=deriv_eq1(an_eq,'x');eqd=rebuild_eq(deriv)// note that the function can be somehow simplified:eqds=rebuild_eq(simplify_eq(deriv))