Matlab Symbolic Toolbox: assumptions not considered for expand function

I have encountered the following inconsistency while programming with the Matlab Symbolic Toolbox.

From trigonometry we know that cos(a*x) = a*cos(x) if 'a' is a variable that can assume only values +1 and -1 (the same holds for 'sin' function). However, this equivalence is not considered by function 'expand', being values of variable 'a' properly constrained. Here is the example:

____________________________

syms a x

assume(a, 'integer'); assumeAlso(a == +1 | a == -1);

s = cos(a*x);

expand(s)

ans =

cos(a*x)

____________________________

How can I include such "rule" in developing symbolic mathematical expressions?