Member predicate MetricElement :: getALevel
An element has no level defined if it is cyclically dependent on itself. Otherwise, it has:
- level 0, if it does not depend on any other elements,
- level 1, if it does not directly depend on another element that occurs in the source, and
- level n+1, if it depends on another element at level n.
Note that according to this definition, an element may have multiple levels.