Action operator -- does not necessary means Spark action; but does mean running BLAS optimizer and writing down Spark graph lineage since last checkpointed DRM.
Distributed context, can be implicitly converted to operations on org.apache.mahout.math.drm. DistributedEngine.
Explicit extraction of key class Tag since traits don't support context bound access; but actual implementation knows it
R-like syntax for number of columns
R-like syntax for number of rows.