{ An integrity constraint on a schema S is a formula I over theunderlying type system with free variables f r(I ) fC1; : : : ; Cng, where each classname Ci is used as a variable of type f(ident : I D; value : TCi)g.{ An instance D of a schema is said to be consistent i substituting D(C ) for eachclass variable C in each integrity constraint I evaluates to true, when interpretedin the usual way.
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