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