@techreport{
    b20,
    author = "Lassila, Eero",
    title = "A Tree Expansion Formalism for Generative String Rewriting",
    number = "HUT-TCS-B20",
    month = "June",
    flags = "copy group_2",
    groups = "GROUP_1 group_2",
    year = "2001",
    institution = "Helsinki University of Technology, Laboratory for Theoretical Computer Science"
}