If the sentence has a token at position tokenIndex, then this returns the
singleton sequence containing that token.
If the sentence has a token at position tokenIndex, then this returns the
singleton sequence containing that token. Otherwise, this returns an empty sequence.
the sentence
the index of the desired token in the sentence
a singleton sequence containing the specified token (if it exists); otherwise an empty sequence
The KeywordFeature maps a token to its word representation, if its word appears in the argument set
keywords. Otherwise its apply function will return an empty set.See the definition of TokenFeature (above) for more details about the interface.