Research proof index

Reconstruction of a tensor from the evaluations of its first index: every t : Tensor S c is the sum over basis indices i of the basis covector basis ![c 0] (single.symm i) tensored with the evaluation evalT 0 i t, with the prepended index permuted back into the first slot. This is the first-index analogue of eq_sum_evalT.

Commuting evaluation with permutations.

Evaluating an index in the right factor of a tensor product commutes with forming the product, up to the identity reindexing which identifies the two ways of removing that index from the appended color list.