1. A method of resolving user input that crosses different application domains to identify a discourse item, the method comprising: matching a first portion of the user input to a first grammar rule comprising a description of a first surface semantic structure within output tags; matching a second portion of the user input to a second grammar rule comprising a description of a second surface semantic structure within output tags; constructing the first surface semantic structure; constructing the second surface semantic structure; modifying the constructed first surface semantic structure based on the constructed second surface semantic structure to form an output surface semantic structure; constructing a discourse semantic structure based on the output surface semantic structure, the discourse semantic structure comprising information between tags representing an entity; a processor providing a discourse item definition associated with a first application, the discourse item definition comprising at least one slot for a defining entity that is to be used to identify a discourse item, the slot providing an entity type and a name for the defining entity, the discourse item definition further comprising an identification of a first domain expert; a processor providing an entity definition associated with a second application, the entity definition having a type that matches the entity type provided by the slot and a name that differs from the name provided by the slot and the entity definition defining an entity that can fulfill the role of the defining entity in the discourse item definition, the entity definition further comprising an identification of a second domain expert different from the first domain expert; identifying an entity based on the entity definition and at least a portion of the user input by passing a reference to a node in the discourse semantic structure to the second domain expert, which identifies the entity based in part on information in the discourse semantic structure; updating the discourse semantic structure by inserting the identified entity in the discourse semantic structure by replacing the information representing the entity with the identified entity; and identifying a discourse item based on the discourse item definition and the identified entity by passing a reference to a node in the discourse semantic structure to the first domain expert, which identifies the discourse item based in part on the identified entity.