First, we make our way down to the base case:

mystery([7, -2, -8, 10])
------------------------
    values = [7, -2, -8, 10]
    myst_rest = mystery([-2, -8, 10]) = ...
    return ...

    mystery([-2, -8, 10])
    ---------------------
        values = [-2, -8, 10]
        myst_rest = mystery([-8, 10]) = ...
        return ...

        mystery([-8, 10])
        ---------------------
            values = [-8, 10]
            myst_rest = mystery([10]) = ...
            return ...

            mystery([10])
            -------------
                values = [10]
                myst_rest = mystery([]) = ...
                return ...

                mystery([])
                -----------
                    values = []
                    return []

Then, we work our way back through the sections for the previous calls,
which gives the following complete answer:

mystery([7, -2, -8, 10])
------------------------
    values = [7, -2, -8, 10]
    myst_rest = mystery([-2, -8, 10]) = [10, -8, -2]
    return [7] + [10, -8, -2] = [7, 10, -8, -2]

    mystery([-2, -8, 10])
    ---------------------
        values = [-2, -8, 10]
        myst_rest = mystery([-8, 10]) = [10, -8]
        return [10, -8] + [-2] = [10, -8, -2]

        mystery([-8, 10])
        ---------------------
            values = [-8, 10]
            myst_rest = mystery([10]) = [10]
            return [10] + [-8] = [10, -8]

            mystery([10])
            -------------
                values = [10]
                myst_rest = mystery([]) = []
                return [10] + [] = [10]

                mystery([])
                -----------
                    values = []
                    return []