Question: Is there a way to evaluate function like this ?

I'm trying to evaluate function like this , but  D(x) always returns 0, is  there any way to evaluate the function ? 

 

f := proc (x) options operator, arrow, function_assign; piecewise(x < 0, 3^(-(1/3)*x^2), .5*2^(5^(-x))) end proc;
R(x) := int(f(t), t = x .. x+l):
L(x) := int(f(t), t = x-l .. x):
D(x) := (R(x)-L(x))/(R(x)+L(x)):

 

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