-- List of all possible variable names before parsing expression
all_numbers = 1 : map (+1) all_numbers
str_numbers = map (++ "_") (map show all_numbers)
-- get a list of all of our free vars
freeVarslst exp = toList $ fromList (foldl (++) [] (map freeVars exp))
-- now remove all free vars from our list of vars
freshVars exp = filter  (\x -> notElem x (freeVarslst exp)) str_numbers