4. The search engine of claim 1 , further configured to use the ranking function to rank each snippet using one or more ranking features including one or more of a first feature associated with the number of words in each snippet, a second feature associated with the number of punctuation symbols in each snippet, a third feature associated with the number of numbers in each snippet, and a fourth feature associated with the score of the proximity feature.
4. The search engine of claim 1 , further configured to use the ranking function to rank each snippet using one or more ranking features including one or more of a first feature associated with the number of words in each snippet, a second feature associated with the number of punctuation symbols in each snippet, a third feature associated with the number of numbers in each snippet, and a fourth feature associated with the score of the proximity feature. 6. The search engine of claim 4 , further configured to calculate the score of a span for the proximity feature based in part on one or more of a tunable proximity factor, a number of query terms in the span, and a number of terms in the span.