关于“字母算术”的问题，求解释

import re
import itertools

def solve(puzzle):
    words = re.findall('[A-Z]+',puzzle.upper())
    unique_characters = set(''.join(words))
    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) +\
                        ''.join (unique_characters - first_letters)
    characters = tuple(ord(c) for c in sorted_characters)
    digits = tuple(ord(c) for c in '0123456789')
   
    zero = digits[0]
    for guess in itertools.permutations(digits,len(characters)):
         if zero not in guess[:n]:
            equation =puzzle.translate(dict(zip(characters,guess)))
            if eval(equation):
                return equation

if __name__=='__main__':
  　import sys
　  for puzzle in sys.argv[1:]:
     　　print(puzzle)
     　　solution = solve(puzzle)
　　　　 if solution:
      　　　print(solution)　

求红色字体的代码用意？为啥要这么写这个条件？
谢谢．．．