@article{Clark:1987,
 author = "Adrian Clark",
 title = "Halftone output from {\TeX}",
 journal = TUB,
 volume = "8",
 number = "3",
 year = "1987",
 pages = "270--274"
}

@unpublished{Clark:1989,
 author = "James Clark",
 title = "{DVITOPS} User Manual",
 year = "1989",
 note = "unpublished machine-readable documentation"
}

@article{Clark:1991,
 author = "Adrian Clark",
 title = "Practical halftoning with {\TeX}",
 journal = TUB,
 volume = "12",
 number = "1",
 year = "1991",
 pages = "157--165"
}