A software tool unveiled Friday aims to help online firms quickly find and eliminate extremist content used to spread and incite violence and attacks. The Counter Extremism Project, a nongovernment group based in Washington, proposed its software be used in a system similar to one used to prevent the spread on online child pornography. "We think this is the technological solution to combat online extremism," said Mark Wallace, chief executive of the organization that includes former diplomats and public officials from the United States and other countries.