1. A method performed by one or more computers, the method comprising: accessing data indicating a set of classes corresponding to a concept, each of the classes representing a different set of words or phrases associated with the concept; generating a first language model in which a first class from the set of classes represents the concept; generating a second language model in which second classes from the set of classes represent the concept, each of the second classes being different from the first class; obtaining (i) output of the first language model for a set of input data, and (ii) output of the second language model for the set of input data; evaluating the output of the first language model and the output of the second language model; and selecting one or more classes from the set of classes based on evaluating the output of the first language model and the output of the second language model.