added difference_of_simplified_exponential_functions function

src/main/py/read_math_function.py

@@ -117,4 +117,8 @@ def sum_of_simplified_exponential_functions(type, function):
     if type !="se":
         return False
     summands = function.split('+')
-    return simplify_exponential_function('e', summands)
+    return simplify_exponential_function('e', summands)
+
+def difference_of_simplified_exponential_functions(type, function):
+    if type !="de":
+        return False