Whitespace and link reference definitions


I do not understand something about link reference definitions. The spec says that there can be an optional whitespace between the link label and the link destination, and another optional whitespace between the link destination and the optional link title, but to me the reference implementations are only accepting spaces and tabulations.
For example, “[foo]: /url” should be parsed as a link reference definition because " " (a space and two form feeds) is a whitespace (isn’t it ?), but it is parsed as a paragraph by the C implementation and the interactive dingus.
Could somebody please explain it to me ?
(Sorry for my bad english ^^)