Zero-One-Infinity Rule prov.   "Allow none of {foo}, one of
   {foo}, or any number of {foo}."   A rule of thumb for software
   design, which instructs one to not place {random} limits on the
   number of instances of a given entity (such as: windows in a window
   system, letters in an OS's filenames, etc.).   Specifically, one
   should either disallow the entity entirely, allow exactly one
   instance (an "exception"), or allow as many as the user wants -
   address space and memory permitting.
  
      The logic behind this rule is that there are often situations where
   it makes clear sense to allow one of something instead of none.
   However, if one decides to go further and allow N (for N > 1), then
   why not N+1?   And if N+1, then why not N+2, and so on?   Once above
   1, there's no excuse not to allow any N; hence, {infinity}.
  
      Many hackers recall in this connection Isaac Asimov's SF novel
   "The Gods Themselves" in which a character announces that the number
   2 is impossible - if you're going to believe in more than one
   universe, you might as well believe in an infinite number of them.
  
  

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by the DICT Development Group