14. At least one computer-readable storage medium, the at least one computer-readable storage medium being an article of manufacture and comprising computer-executable instructions that, when executed, perform a method of operating a document authoring tool that authors an interactive document, the method comprising: receiving user input defining elements of the interactive document, comprising: a plurality of entities to be incorporated in the interactive document; a transformation indicating a relationship between input data and output data of a first entity of the plurality of entities; an interactive graphical object for visually representing input data of a second entity of the plurality of entities, wherein the document authoring tool selects the interactive graphical object which is represented by a third entity and a fourth entity, the third entity comprising an entity with an input and no output and the fourth entity comprising an entity with an output and no input, the third entity being separate and disconnected from the fourth entity; an entity representing a source of external data; a set of equations; and a plurality of edges, each of the edges indicating a data flow between two of the plurality of entities; and verifying that the plurality of edges interconnect the plurality of entities in an acyclic graph.
14. At least one computer-readable storage medium, the at least one computer-readable storage medium being an article of manufacture and comprising computer-executable instructions that, when executed, perform a method of operating a document authoring tool that authors an interactive document, the method comprising: receiving user input defining elements of the interactive document, comprising: a plurality of entities to be incorporated in the interactive document; a transformation indicating a relationship between input data and output data of a first entity of the plurality of entities; an interactive graphical object for visually representing input data of a second entity of the plurality of entities, wherein the document authoring tool selects the interactive graphical object which is represented by a third entity and a fourth entity, the third entity comprising an entity with an input and no output and the fourth entity comprising an entity with an output and no input, the third entity being separate and disconnected from the fourth entity; an entity representing a source of external data; a set of equations; and a plurality of edges, each of the edges indicating a data flow between two of the plurality of entities; and verifying that the plurality of edges interconnect the plurality of entities in an acyclic graph. 17. The computer-readable storage medium of claim 14 , wherein: receiving a user input defining an external data source comprises receiving user input defining a query-able source; and the method further comprises recording as part of the interactive document information indicating a query to the query-able data source.