Bracktracking for KnowledgeBase answering

[sumedhghaisas]

Now that data types Fact and Clause supports location tracking a verbose backtrack of knowledge base answer can be generated. Backtrack of the answer will print the stack, basically which queries are made at which level. Printing the substitution achieved at each level is doubtful as now those pointers may have taken different values, but I am not sure. This will be useful for debugging purpose.