1. A method comprising: calculating, by one or more processors, a first value derived from coherence of terms in a sequence, where the coherence of the terms in the sequence is calculated relative to a first collection of documents; calculating, by one or more processors, a second value derived from a variation of context in which the sequence occurs, where the variation of context is calculated relative to a second collection of documents that differs from the first collection of documents; determining, by one or more processors, that the sequence is a semantic unit based on the first value, the second value, and one or more rules for excluding particular sequences; and labeling, by one or more processors, the sequence as a semantic unit.