*I need to export an svm I trained with Classify to the standard SVM Format (which is readable by libsvm).*

The SVM format obeys the following Backus-Naur Form (see 
http://www.cs.cornell.edu/People/tj/svm_light/), it looks like this:

    <line> ::= <target> " " (<feature> ":" <value>)+
    <target> ::= <positive-integer>
    <feature> ::= <positive-integer>
    <value> ::= <float>


How can I transform the internal representation of Mathematica's Classifiers into this representation?

**Notes:**

 - MATLAB provides libsvmread to import svms. 
 - In python sklearn.datasets.load_svmlight_file imports svms.
 - Libsvm uses the so called "sparse" format where zero values do not need to be stored.

  [1]: http://www.csie.ntu.edu.tw/~cjlin/libsvm/#matlab