This is also how it works in Julia, where macros digest notation for einsum-like...

This is also how it works in Julia, where macros digest notation for einsum-like operations before compile-time. In fact the linked file's explanatory comment:

     (einsum (A i j) (B i k) (C k j)) 
    results in the the updates
      A[i,j] = \\sum_k B[i,k]C[k,j],
    which is equivalent to matrix multiplication.

very nearly contains the syntax used by all the Julia packages (where @ marks a macro), which is

    @tensor A[i,j] = B[i,k] * C[k,j]

(using https://github.com/Jutho/TensorOperations.jl, but see also OMEinsum, Finch, and my Tullio, TensorCast.)