Pick up that can.</small></p>
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<p>If we store pointers to values, rather than copies, we need to ensure that the data pointed to is not deleted before some other value tries to make use of it. We want to delete it when there are no longer any references to it. One method to do this, called <em>Mark and Sweep</em>, is to monitor those values that are in the environment, as well as every value that has been allocated. When a variable is put into the environment it, and everything it references, is <