abs and sign simplification

In some symbolic manipulations, I get a result which has something like abs(e)*sign(e). Why is this not simplified away, even by simplify? It's surely an identity that e = abs(e)*sign(e). For example, with this code:

syms x
diff(norm(diff(cos(x))))
% ans = (abs(sin(t))*sign(sin(t))*cos(t))/(abs(sin(t))^2)^(1/2)

Answer 1

[Massimo Zanetti]

It happens because for the scalar case norm(y)=|y| (here y=y(x) is any function), therefore Matlab computes derivative by splitting the two cases in the derivative of|y|: y>=0 and y<0. Then it combines the results in a signle function by the sign*|.| trick.

[Corey Richardson]

OK, that makes sense. Is there a way to rewrite them away?

[Massimo Zanetti]

Actually the point is, why you need to rewrite something that is correct?

[Corey Richardson]

For display purposes in live scripts, it muddies the expressions.