Feature functions are factorized as products of two indicator functions: one defined on the input sequence (input features) and the other on the output labels (output features).
we could re-write For gap segments, we consider the following input feature templates: N-GRAM: Indicator function for n-gram appeared in the segment (n = 1, 2) ANCHOR: Indicator function for its relative position to the event anchor words (to the left, to the right, overlaps, contains) and the following output feature templates: 1STORDER: Indicator function for the combination of its immediate left argument and its immediate right argument.