However, Common Lisp’s lexical variables are lexical variables with a twist, at least compared to the original Algol model. The twist is provided by the combination of lexical scoping with nested functions. By the rules of lexical scoping, only code textually within the binding form can refer to a lexical variable. But what happens when an anonymous function contains a reference to a lexical variable from an enclosing scope? For instance, in this expression:

The key thing to understand about closures is that it’s the binding, not the value of the variable, that’s captured. Thus, a closure can not only access the value of the variables it closes over but can also assign new values that will persist between calls to the closure. For instance, you can capture the closure created by the previous expression in a global variable like this:

A single closure can close over many variable bindings simply by referring to them. Or multiple closures can capture the same binding. For instance, the following expression returns a list of three closures, one that increments the value of the closed over count binding, one that decrements it, and one that returns the current value: