e system 
</li><li>explanation facilities to explain how a goal was proved, why an
answer couldn't be found, why a question was asked, why an
error-producing goal was called, and why the depth-bound was reached 
</li><li>knowledge-level debugging tools, that let the user debug
incorrect answers, missing answers, why the system asks a question,
system errors, and possible infinite loops
</li><li>depth-bounded search, that can be used to investigate potential
infinite loops and used to build an iterative-deep