missing feature tf.einsum() https://github.com/tensorflow/tensorflow/blame/747a44448768d1d50322b6100c45ca7374d40e88/tensorflow/python/ops/special_math_ops.py https://github.com/tensorflow/tensorflow/blame/7334db370cfc2272e1b508d7fe1c059ec97e65bc/tensorflow/python/ops/special_math_ops.py#L1170
