| 11. | However, a closure requires that the free variables it references survive the enclosing function's execution.
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| 12. | The free variables of the expression must also be free where the function is defined.
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| 13. | Start by converting the free variable to an argument:
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| 14. | Suppose that is a formula with one free variable.
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| 15. | Higher-order patterns are lambda-terms where the arguments of a free variable are all distinct bound variables.
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| 16. | Computing with non-deterministic operations and computing with free variables by narrowing has the same expressive power.
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| 17. | These free variables are implicitly considered universally quantified.
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| 18. | A formula in first-order logic with no free variables is called a "'first-order sentence " '.
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| 19. | Thus, one is motivated to somehow track the occurrences of the free variables in the expression.
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| 20. | However, in mathematics, an expression with no free variables must have one and only one value.
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