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Move characters from the output sequence to the input sequence.
void commit( std::size_t n);
          Appends n characters from
          the start of the output sequence to the input sequence. The beginning of
          the output sequence is advanced by n
          characters.
        
          Requires a preceding call prepare(x)
          where x >=
          n, and no intervening operations
          that modify the input or output sequence.
        
          If n is greater than the
          size of the output sequence, the entire output sequence is moved to the
          input sequence and no error is issued.