@srowen Do you think we should change the format from 

~~~
<L>,<f1> <f2> ...
~~~

to

~~~
<L> <f1> <f2> ...
~~~

(replace `,` by ` `) to be consistent with LIBSVM format. And eventually we may support multi-label, in which case we can use

~~~
<l1>,<l2> <f1> <f2>
<l1>,<l2>,<l3> <f1> <f2>
~~~