  It is known that, for given integers s \geq 0 and j > 0, the nested recursion
R(n) = R(n - s - R(n - j)) + R(n - 2j - s - R(n - 3j)) has a closed form
solution for which a combinatorial interpretation exists in terms of an
infinite, labeled tree. For s = 0, we show that this solution sequence has a
closed form as the sum of ceiling functions C(n). Further, given appropriate
initial conditions, we derive necessary and sufficient conditions on the
parameters s1, a1, s2 and a2 so that C(n) solves the nested recursion R(n) =
R(n - s1 - R(n - a1)) + R(n- s2 - R(n - a2)).
