Slicing the DRM.
Row range. This must be '::' (all-range) unless matrix rows are keyed by Int key.
col range. Must be a sub-range of
0 until ncol. '::' denotes all-range.
Map matrix block-wise vertically.
new matrix' width (only needed if width changes).
If changed from default, ensures the product has at least that much parallelism.
if changed from default, ensures the pipeline product has exactly that much parallelism.
If changed from default, engine-specific automatic parallelism adjustment strategy is applied.