1class RunewordsCalculator:
 2    def __init__(self, runewords):
 3        self.runewords = list(runewords.items())
 4        self.runes = []
 5        self.used = set()
 6
 7    def add_runes(self, runes):
 8        self.runes.extend(runes)
 9
10    def __iter__(self):
11        return self
12
13    def __next__(self):
14        if len(self.used) == len(self.runewords):
15            raise StopIteration
16
17        for name, recipe in self.runewords:
18            if name in self.used:
19                continue
20
21            if self._can_build(recipe):
22                self.used.add(name)
23                return name
24
25        return None
26
27    def _can_build(self, recipe):
28        i = 0
29        for rune in self.runes:
30            if rune == recipe[i]:
31                i += 1
32                if i == len(recipe):
33                    return True
34        return False