9. A system comprising: one or more computers each comprising one or more processors; and one or more storage devices storing instructions that are configured to, when executed by the one or more computers, cause the one or more computers to perform operations comprising: receiving a specification of sections that defines a first section, a second section, and a connecting section, wherein the first section identifies a first search engine and the second section identifies a second search engine, the second search engine being different from the first search engine, and wherein the connecting section defines a condition and an operation, wherein the condition determines when to combine results obtained from the first search engine and the second section, and the operation identifies how to combine the results; receiving a search query; generating a first output query and a second output query, wherein each output query is based on the received search query; submitting the first, output query to the first search engine and the second output query to the second search engine; receiving first ordered results from the first search engine in response to the first output query; receiving second ordered results from the second search engine in response to the second output query; combining the first ordered results and the second ordered results based on the condition and the operation to form combined results; and outputting a representation of the combined results as a response the search query.
9. A system comprising: one or more computers each comprising one or more processors; and one or more storage devices storing instructions that are configured to, when executed by the one or more computers, cause the one or more computers to perform operations comprising: receiving a specification of sections that defines a first section, a second section, and a connecting section, wherein the first section identifies a first search engine and the second section identifies a second search engine, the second search engine being different from the first search engine, and wherein the connecting section defines a condition and an operation, wherein the condition determines when to combine results obtained from the first search engine and the second section, and the operation identifies how to combine the results; receiving a search query; generating a first output query and a second output query, wherein each output query is based on the received search query; submitting the first, output query to the first search engine and the second output query to the second search engine; receiving first ordered results from the first search engine in response to the first output query; receiving second ordered results from the second search engine in response to the second output query; combining the first ordered results and the second ordered results based on the condition and the operation to form combined results; and outputting a representation of the combined results as a response the search query. 12. The system of claim 9 , wherein the specification of sections comprises a tree data structure having a first node representing the connecting section, a first child node of the first node representing the first section, and a second child node of the first node representing the second section.