parameters: (src_sents, tgt_sents, vocab_size, freq_cutoff)
return_statement: return Vocab(src, tgt)
docstring: Build Vocabulary.
          @param src_sents (list[str]): Source sentences provided by read_corpus() function
          @param tgt_sents (list[str]): Target sentences provided by read_corpus() function
          @param vocab_size (int): Size of vocabulary for both source and target languages
          @param freq_cutoff (int): if word occurs n < freq_cutoff times, drop the word.
docstring_summary: Build Vocabulary.
docstring_tokens: Build Vocabulary .
function: def build(src_sents, tgt_sents, vocab_size, freq_cutoff) -> 'Vocab':
          """ Build Vocabulary.
          @param src_sents (list[str]): Source sentences provided by read_corpus() function
          @param tgt_sents (list[str]): Target sentences provided by read_corpus() function
          @param vocab_size (int): Size of vocabulary for both source and target languages
          @param freq_cutoff (int): if word occurs n < freq_cutoff times, drop the word.
          """
          assert len(src_sents) == len(tgt_sents)
  
          print('initialize source vocabulary ..')
          src = VocabEntry.from_corpus(src_sents, vocab_size, freq_cutoff)
  
          print('initialize target vocabulary ..')
          tgt = VocabEntry.from_corpus(tgt_sents, vocab_size, freq_cutoff)
  
          return Vocab(src, tgt)
function_tokens: def build ( src_sents , tgt_sents , vocab_size , freq_cutoff ) -> 'Vocab' : assert len ( src_sents ) == len ( tgt_sents ) print ( 'initialize source vocabulary ..' ) src = VocabEntry . from_corpus ( src_sents , vocab_size , freq_cutoff ) print ( 'initialize target vocabulary ..' ) tgt = VocabEntry . from_corpus ( tgt_sents , vocab_size , freq_cutoff ) return Vocab ( src , tgt )