def fill_missing_words(grammar: PCFG, missing_words: Set[str]):
# UNK -> word1 | word2 | ... | wordN
unknown = Nonterminal('UNK')
unk_rules = [
Production(unknown, [missing_word]) for missing_word in missing_words
]
# Add UNK as a possibility to all rules with strings in the right hand side
corrected_rules: List[Nonterminal] = []
rule: ProbabilisticProduction
for rule in grammar.productions():
# right hand side has a string somewhere
if any(isinstance(element, str) for element in rule.rhs()):
# rule has already been corrected
if rule.lhs() in corrected_rules:
continue
unk_rules.append(Production(rule.lhs(), [unknown]))
corrected_rules.append(rule.lhs())
return induce_pcfg(grammar.start(), grammar.productions() + unk_rules)
