Abdolreza Hatamlou and  Yusef Farhang and  Mohammad Reza Meybodi,  Combining Variable Ordering Heuristics for Improving Search Algorithms Performance.   journal   = {International Journal of Computer and Information Engineering}, [online]. World Academy of Science, Engineering and Technology.
    February 2009, vol. 26(2). 427 - 429
    [viewed 30 January 2020]. Available from: https://publications.waset.org/pdf/2235.