@Article{Mar-Son:occur89,
  author       	= {K. Marriott and H. S{\o}ndergaard},
  title        	= {On {Prolog} and the Occur Check Problem},
  journal      	= {SIGPLAN Notices},
  volume       	= {24},
  number       	= {5},
  pages        	= {76--82},
  year         	= {1989},
  abstract	= {It is well known that omission of the occur check in 
		   unification leads to unsound Prolog systems.  Nevertheless,
	 	   most Prolog systems omit the occur check because this
		   makes unification much faster and unsoundness allegedly
		   seldom manifests itself.  We revisit the occur check
		   problem and point to two aspects that have previously 
		   received no attention.  Firstly, ``unification without the
		   occur check'' is ambiguous, and in practice, Prolog systems
		   vary markedly in their reaction to programs having occur
		   check problems.  Secondly, even very simple program
		   transformations are unsafe for pure Prolog when the occur
		   check is omitted.  We conclude that the occur check problem
		   is important, and in particular, that the current effort to
		   standardize Prolog should address it.  },
}