The Pentagon, the world's largest user of drones, has posted a new policy on signs outside the mammoth five-sided building: No Drone Zone. The signs, complete with a red slash through an image of a quadcopter drone, reflect America's growing concern about the proliferation of the small, inexpensive remote-controlled devices and the risk they pose to safety, security and privacy. Federal law prohibits flying a drone anywhere in and around Washington, an area known as the National Capital Region. Other communities and institutions across the country are wrestling with the potential threat from more than 400,000 private and commercial drones now registered to operate in the skies. The pilot of a commercial jetliner said his plane nearly collided with a drone while approaching Los Angeles International Airport on Friday afternoon, sparking a search by L.A. police and sheriff's officials for the owner of the unmanned aircraft.

The proliferation of online information sources has led to an increased use of wrappers for extracting data from Web sources. While most of the previous research has focused on quick and efficient generation of wrappers, the development of tools for wrapper maintenance has received less attention. This is an important research problem because Web sources often change in ways that prevent the wrappers from extracting data correctly. We present an efficient algorithm that learns structural information about data from positive examples alone. We describe how this information can be used for two wrapper maintenance applications: wrapper verification and reinduction. The wrapper verification system detects when a wrapper is not extracting correct data, usually because the Web source has changed its format. The reinduction algorithm automatically recovers from changes in the Web source by identifying data on Web pages so that a new wrapper may be generated for this source. To validate our approach, we monitored 27 wrappers over a period of a year. The verification algorithm correctly discovered 35 of the 37 wrapper changes, and made 16 mistakes, resulting in precision of 0.73 and recall of 0.95. We validated the reinduction algorithm on ten Web sources. We were able to successfully reinduce the wrappers, obtaining precision and recall values of 0.90 and 0.80 on the data extraction task.