prod -- matrix-matrix product
    -1 -- default implementation, float
    -2 -- default implementation, complex<float>
    -3 -- generic implementation, float
    -4 -- generic implementation, complex<float>
    -5 --    BLAS implementation, float
    -6 --    BLAS implementation, complex<float> {interleaved only}
   -11 -- default impl with transpose, float
   -12 -- default impl with transpose, complex<float>
   -13 -- default impl with hermetian, complex<float>
  -103 -- generic impl, plain blocks, float
  -104 -- generic impl, plain blocks, complex<float>