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x is prime : <=> y not | xis not a definition, because free y does not occur on left side.
x is a divisor of a*y, if x*z = a*y, for some z.is not a definition, because on the left hand side of the formula
x is a divisor of a*y <=> exists z: x*z = a*y.a general term a*y appears where only a variable is allowed.
Beware of bogus definitions!