Why simple expression is not simplified (Symbolic Toolbox)?

Quite simple code

syms a b
assume(a,'real');
assume(b,'real');
simplify((-1)^a*(b-1)^a)

The answer is obviously (1-b)^a, but Matlab does not simplify it? Why? How to make it work?

Thank you!

Answer 1

[Walter Roberson]

That is not a correct solution.

Let b = -31, let a = 3/5

simplify((1-(-31))^(3/5));
                               8
simplify((-1)^(3/5)*((-31)-1)^(3/5));
                                 (1/5)
                          -8 (-1)     
evalf(%);
                  -6.472135955 - 4.702282018 I

[Alex]

But what assumption should I provide to Matlab to make this simplified to (1-b)^a?

[Walter Roberson]

I am not sure. The obvious ones do not work.