@article{Ehrlich1973a, 
     author = "Ehrlich, Gideon", 
     doi = "10.1145/321765.321781", 
     issn = "00045411", 
     journal = "Journal of the ACM", 
     link = "http://dl.acm.org/citation.cfm?id=321765.321781", 
     month = "jul", 
     number = "3", 
     pages = "500--513", 
     publisher = "ACM", 
     title = "Loopless Algorithms for Generating Permutations, Combinations, and Other Combinatorial Configurations", 
     uri = "http://dl.acm.org/citation.cfm?id=321765.321781", 
     volume = "20", 
     year = "1973", 
}

Problems: 405