1. A method of determining policy applicability, comprising: designating a document as a source document representative of a policy to which applicability of a potential target document is to be determined; responsive to designating the document as the source document representative of the policy, generating, by a processor, a first sketch based on content of the source document, such that the first sketch is not generated until the document is designated as the source document representative of the policy; defining, by the processor, a matching criterion based on the first sketch; generating, by the processor, a second sketch based on content of the potential target document; determining, by the processor, whether the policy applies to the potential target document based on whether the second sketch satisfies the matching criterion, such that whether the policy applies to the potential target document is determined based on the source document insofar as the matching criterion is determined based on the first sketch and insofar as the first sketch is generated based on the content of the source document; and if the policy applies, at least one of (1) automatically providing a notification regarding applicability of the policy and (2) automatically blocking an action so as to prevent a violation of the policy, by the processor, wherein the first sketch is generated in a first manner that is computationally more expensive than a second manner in which the second sketch is generated, wherein the first manner and the second manner are such that the first sketch generated in the first manner is more accurate of the content of the source document than the second sketch generated in the second manner is of the content of the target document, and wherein the first manner and the second manner are such that the first sketch generated in the first manner contains more information than the second sketch generated in the second manner does.
5. A method according to claim 1 , wherein the matching criterion specifies a permissible deviation from the first sketch.