@BOOK{plfa20.07,
    author = {Philip Wadler and Wen Kokke and Jeremy G. Siek},
    title  = {Programming Language Foundations in {A}gda},
    year   = {2020},
    month  = jul,
    url    = {http://plfa.inf.ed.ac.uk/20.07/},
}

@BOOK{plfa19.08,
    author = {Philip Wadler and Wen Kokke},
    title  = {Programming Language Foundations in {A}gda},
    year   = {2019},
    month  = aug,
    url    = {http://plfa.inf.ed.ac.uk/19.08/},
}