added example of human-readable message in assert statement

examples/alphametics.py

@@ -10,7 +10,7 @@ import itertools
 def solve(puzzle):
     words = re.findall('[A-Z]+', puzzle.upper())
     unique_characters = set(''.join(words))
-    assert len(unique_characters) <= 10
+    assert len(unique_characters) <= 10, 'Too many letters'
     first_letters = {word[0] for word in words}
     n = len(first_letters)
     sorted_characters = ''.join(first_letters) + \