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Relations

Definition: relation

R is a relation between S₀, ..., S_n-1 : <=>
   R subset S₀ x ... x S_n-1.

R is an n-ary relation : <=>
   exists S_0, ..., S_n-1: R  is a relation between  S_0, ..., S_n-1;

R is an n-ary relation on S : <=>
   R subset S x ... x S (cartesian product of n instances of S).

R is a relation on S : <=> R is a 2-ary ( binary) relation on S.

Set of tuples of related values.