Loading an ontology that contains cycles in its class structure leads to
ungraceful handling in Swoop.  Try 

http://www.mindswap.org/~tw7/work/survey/cycle.owl

If a new ontology is created and a cycle is added (using told reasoner),
stackoverflowerror occurs when trying to create classTree.

A possible solution is to display the tree as is, but as soon as a cycle is
detected, the last node in the cycle will cease to generate its children.  In
addition, the node's name will read "Cycle from X", where X is the 'head' of the
cycle.