@Article {bds.tissec02,
    author = {P. Bonatti and S. {De Capitani di Vimercati} and P. Samarati},
    title = {An Algebra for Composing Access Control Policies},
    journal = {ACM Transactions on Information and System Security (TISSEC)},
    volume = {5},
    number = {1},
    pages = {1-35},
    month = {February},
    year = {2002}
}