Software that aids in "creating" crossword puzzles has been written since at least 1976; one popular example was Crossword Magic for the Apple II in the 1980s. The earliest software relied on people to input a list of fill words and clues, and automatically maps the answers onto a suitable grid. This is a search problem in computer science because there are many possible arrangements to be checked against the rules of construction. Any given set of answers might have zero, one, or multiple legal arrangements. Modern open source libraries exist that attempt to efficiently generate legal arrangements from a given set of answers.