Add an assertion for the assumption that all identifier start characters are...

Add an assertion for the assumption that all identifier start characters are also valid continuation characters, so that we notice it if that ever changes.

bin/cython-generate-lexicon.py

@@ -81,7 +81,9 @@ def get_continue_characters_as_number():
 def get_continue_not_start_as_number():
     start = get_start_characters_as_number()
     cont = get_continue_characters_as_number()
-    return sorted(set(cont) - set(start))
+    assert set(start) <= set(cont), \
+        "We assume that all identifier start characters are also continuation characters."
+    return sorted(set(cont).difference(start))
 
 
 def to_ranges(char_num_list):