Function ngrams 

pub fn ngrams<'a>(tokens: &[&'a str], n: usize) -> CoreResult<Vec<Vec<&'a str>>>

Generate word-level n-grams from a list of tokens.

Example

use scirs2_core::string_ops::ngrams;

let tokens = vec!["the", "quick", "brown", "fox"];
let bigrams = ngrams(&tokens, 2).expect("bigrams");
assert_eq!(bigrams.len(), 3);
assert_eq!(bigrams[0], vec!["the", "quick"]);