15. A system for processing references, the system comprising: a processor; and a memory storing instructions for controlling the processor to perform operations, the operations comprising: receiving a document for use in a sandbox environment, passing the document to a parser that finds one or more references in the document to other resources to yield a list of one or more references, passing the list of the references to a verifier that verifies that a resource associated with each reference in the list of the one or more references exists to yield a list of verified references, and modifying the sandbox environment based on the list of verified references to yield a modified sandbox environment to include the verified resources.
15. A system for processing references, the system comprising: a processor; and a memory storing instructions for controlling the processor to perform operations, the operations comprising: receiving a document for use in a sandbox environment, passing the document to a parser that finds one or more references in the document to other resources to yield a list of one or more references, passing the list of the references to a verifier that verifies that a resource associated with each reference in the list of the one or more references exists to yield a list of verified references, and modifying the sandbox environment based on the list of verified references to yield a modified sandbox environment to include the verified resources. 20. The system of claim 15 , wherein verifying the respective resource comprises opening the respective resource and comparing actual system behavior to expected system behavior.