operator: word-ngrams

    description:
        word-ngrams[k1, k2, ..., kn] ket
        create word ngrams, of specified sizes
        the result is multiplied by the coefficient of the ket
        
    examples:
        word-ngrams[1] |happy to see you>
            |happy> + |to> + |see> + |you>
            
        -- same thing again, but this time the ket has coefficient of 5.1:
        word-ngrams[1] 5.1|happy to see you>
            5.1|happy> + 5.1|to> + 5.1|see> + 5.1|you>

        word-ngrams[1,2,3] |happy to see you>
            |happy> + |to> + |see> + |you> + |happy to> + |to see> + |see you> + |happy to see> + |to see you>

    see also:
        letter-ngrams