added a comment explaining the origin of the rules for diff
had an idea for implementing simplify
added a more focused version of simp2
small tweaks
modified expr trype
added a fold function for expressions
removed old functions for processing unops and binops
wrote diff using fold--the results were kind of messy
added an mli file
reworked recursive diff implementation
added string processing functions
created a better string_of_expr function
Merge branch 'master' of https://github.com/qbeightol/symDiff