Groundness analysis
-------------------

This analysis assigns to each operation the conditions under which
a ground (non-free) result is computed. The analysis results can be
"always ground result", "possibly non-ground result",
or "ground if arguments x1,..,xn are ground".

The idea and details of this analysis can be found in the
[ICLP 2005 paper](https://doi.org/10.1007/11562931_21).