@misc{pre-03-xfarbe,
  author = {A. Preusser},
  title = {{XFARBE} - {Fill} Area with Bicubics},
  url = {http://www.fhi-berlin.mpg.de/grz/pub/xfarbe_doc.html},
  howpublished = {Online document},
  year = 2003,
  comment = {We used this software to generate a figure for the Affine Arithmetic monograph, illustrating level sets of a bivariate function.},
  altkeys = {xfarbe}
}