Golem is an inductive logic programming algorithm developed by Stephen Muggleton and Cao Feng in 1990.[1] It uses the technique of relative least general generalisation proposed by Gordon Plotkin, leading to a bottom-up search through the subsumption lattice.[2] In 1992, shortly after its introduction, Golem was considered the only inductive logic programming system capable of scaling to tens of thousands of examples.[3]

Description

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Golem takes as input a definite program B as background knowledge together with sets of positive and negative examples, denoted   and   respectively. The overall idea is to construct the least general generalisation of   with respect to the background knowledge. However, if B is not merely a finite set of ground atoms, then this relative least general generalisation may not exist.[4] Therefore, rather than using B directly, Golem uses the set   of all ground atoms that can be resolved from B in at most h resolution steps. An additional difficulty is that if   is non-empty, the least general generalisation of   may entail a negative example. In this case, Golem generalises different subsets of   separately to obtain a program of several clauses.[2] Golem also employs some restrictions on the hypothesis space, ensuring that relative least general generalisations are polynomial in the number of training examples. Golem demands that all variables in the head of a clause also appears in a literal of the clause body; that the number of substitutions needed to instantiate existentially quantified variables introduced in a literal is bounded; and that the depth of the chain of substitutions needed to instantiate such a variable is also bounded.[3]

Example

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Assumed family relations

The following example about learning definitions of family relations uses the abbreviations

par: parent, fem: female, dau: daughter, g: George, h: Helen, m: Mary, t: Tom, n: Nancy, and e: Eve.

It starts from the background knowledge (cf. picture)

 ,

the positive examples

 ,

and the trivial proposition true to denote the absence of negative examples.

The relative least general generalisation is now computed as follows to obtain a definition of the daughter relation.

  • Relativise each positive example literal with the complete background knowledge:
     ,
  • Convert into clause normal form:
     ,
  • Anti-unify each compatible [5] pair [6] of literals:
    •   from   and  ,
    •   from   and  ,
    •   from   and  ,
    •   from   and  , similar for all other background-knowledge literals
    •   from   and  , and many more negated literals
  • Delete all negated literals containing variables that don't occur in a positive literal:
    • after deleting all negated literals containing other variables than  , only   remains, together with all ground literals from the background knowledge
  • Convert clauses back to Horn form:
    •  

The resulting Horn clause is the hypothesis h obtained by Golem. Informally, the clause reads "  is called a daughter of   if   is the parent of   and   is female", which is a commonly accepted definition.

References

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  1. ^ Muggleton, Stephen H.; Feng, Cao (1990). Arikawa, Setsuo; Goto, Shigeki; Ohsuga, Setsuo; Yokomori, Takashi (eds.). "Efficient Induction of Logic Programs". Algorithmic Learning Theory, First International Workshop, ALT '90, Tokyo, Japan, October 8-10, 1990, Proceedings. Springer/Ohmsha: 368–381.
  2. ^ a b Nienhuys-Cheng, Shan-hwei; Wolf, Ronald de (1997). Foundations of inductive logic programming. Lecture notes in computer science Lecture notes in artificial intelligence. Berlin Heidelberg: Springer. pp. 354–358. ISBN 978-3-540-62927-6.
  3. ^ a b Aha, David W. (1992). "Relating relational learning algorithms". In Muggleton, Stephen (ed.). Inductive logic programming. London: Academic Press. p. 247.
  4. ^ Nienhuys-Cheng, Shan-hwei; Wolf, Ronald de (1997). Foundations of inductive logic programming. Lecture notes in computer science Lecture notes in artificial intelligence. Berlin Heidelberg: Springer. p. 286. ISBN 978-3-540-62927-6.
  5. ^ i.e. sharing the same predicate symbol and negated/unnegated status
  6. ^ in general: n-tuple when n positive example literals are given