To be fair, CNTK (BrainScript) has quite impressive list of features to support dynamic control structure (in a symbolic fashion, comparing to PyTorch which delegated much of the dynamic control structure to underlying language Python). I think Tensorflow and CNTK probably the only two frameworks pursued such implementation strategy. IMHO, looking back, supporting control structures may not be that useful (see the recent attention based models, all of them can be unroll'ed to ordinary graphs), but it is so interesting to implement!