Kill Assert as we know it for now. Keep the trivial base case.

Assert may not be helping at all. It is trimming crucial information
from the formula, i.e.

[[ a,b ;  a = size(x) AND a+1 = size(b); x ]].

Applying assert will remove 'a+1 = size(b)' from phi and push it as
precondition when reconstructing a solution. The subproblem will become:

[[ a,b ; a = size(x) ; x]]

The input 'b' is then removed by applying Unused-Input.