1. A method executed by a data processing apparatus, comprising: identifying a seed query for a structured document based on a performance of the seed query with respect to the structured document; identifying, by the one or more computers, one or more embedded coding fragments for the structured document using the seed query, each identified embedded coding fragment specifying a structure of a portion of the structured document that includes at least one term of the seed query; generating, by the one or more computers, one or more query templates, each query template corresponding to at least one of the identified embedded coding fragments, the query template including the structure of the corresponding at least one embedded coding fragment and a generative rule to be used in generating one or more synthetic queries; generating, by the one or more computers, the one or more synthetic queries using the one or more query templates and other structured documents, the generating comprising, for each query template: identifying a portion of a particular structured document that includes the structure specified by the corresponding embedded coding fragment; and generating a synthetic query using text contained in the portion of the particular structured document and specified by the generative rule; and storing, by the one or more computers, at least one of the one or more synthetic queries in a query store.