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.
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. 8. The method of claim 1 , where the variation of context in which the sequence occurs, HM(S), is calculated as 
 HM ( S )=MIN( HLM ( S ), HRM ( S )), where MIN defines a minimum operation, HLM(S) is defined as a minimum of 1 - f ⁡ ( wS ) f ⁡ ( S ) for each term w in the second collection of documents, HRM(S) is defined as a minimum of 1 - f ⁡ ( Sw ) f ⁡ ( S ) for each term w in the second collection of documents, f(wS) defines a number of times that a particular term, w, appears in the second collection of documents followed by the sequence, f(Sw) refers to a number of times that the sequence is followed by w in the second collection of documents, and f(S) refers to a number of times that the sequence is present in the second collection of documents.