Cells are data structures similar to pairs except that they have only one element. They are useful for managing state.
Alters the contents of cell to be object. Returns an unspecified value.
Alters the contents of cell to be object, calls thunk with no arguments, then restores the original contents of cell and returns the value returned by thunk. This is completely equivalent to dynamic binding of a variable, including the behavior when continuations are used (see Dynamic Binding).