Translate the feature specification into features.
Translate the feature specification into features. This is done in a short circuiting way so that it stops when the any feature cannot be produced.
model input type
a map of feature name to feature specification
a semantics with which feature specifications should be interpretted.
a mapping from feature name to feature function. Note that the indices matter and that's why we don't want to use a map.
Created by deak on 11/1/15.