These are the current limitations of the Habanero-Java implementation
of the Concurrent Collections programming model.

1. Tags are implemented (and can only be used as) X10 (and HJ) points.
In the CnC specification, tags can (in general) be anything (a string,
a touple, an integer) that can be used to uniquely identify a data
item or a step in a CnC program. In our current CnC implementation,
for simplicity and performance reasons, we have restricted the tags to
points (integer tuples with arbitrary rank).

2. The rank of points used as tags is not checked for uniformity and
completely depends on the user. The user can use a mixture of 1-D, 2-D
and 3-D tags to prescribe steps from the same collection, for
example. This could be considered a feature instead of a limitation,
as it allows the user more freedom in designing the tags for a step collection.
