When sub-indexing an array A by another array B, the sub-indexed array inherits of the shape of its sub-indexer, e.g. B sub-indexes A, then A[B] has the same dimension/shape as B. If the sub-indexed array A is sub-indexed by 2 or more, potentially different, arrays, e.g. B, C, then B and C must be of the same shape and dimension. If there is a shape mismatch (dimensional mismatch, e.g. a 2D and 3D array), then one array must be extended to accommodate the larger array. This is done by the outer-product with the ones array (an array of entirely ones), e.g. for extending Λ Ii :