  A read-once oblivious arithmetic branching program (ROABP) is an arithmetic
branching program (ABP) where each variable occurs in at most one layer. We
give the first polynomial time whitebox identity test for a polynomial computed
by a sum of constantly many ROABPs. We also give a corresponding blackbox
algorithm with quasi-polynomial time complexity $n^{O(\log n)}$. In both the
cases, our time complexity is double exponential in the number of ROABPs.
  ROABPs are a generalization of set-multilinear depth-$3$ circuits. The prior
results for the sum of constantly many set-multilinear depth-$3$ circuits were
only slightly better than brute-force, i.e. exponential-time.
  Our techniques are a new interplay of three concepts for ROABP: low
evaluation dimension, basis isolating weight assignment and low-support rank
concentration. We relate basis isolation to rank concentration and extend it to
a sum of two ROABPs using evaluation dimension (or partial derivatives).
