Given two feature extractors f 1 , f 2 ∈ F, feature addition (denoted by +) produces a feature extractor f 1 + f 2 that adds up the images of f 1 and f 2 .
for any example x ∈ X, we have For example, the feature extractor w + w -1 will map the word alone to a vector that is one for the basis elements w=alone and w -1 =went.