Besides the list constructor cons and the list selectors car and cdr there are also
list mutators called set-car! and set-cdr!
The list mutator procedures:
The command (set-car! x y) changes the value of the car position of the cons cell referred by x.
The car position is assigned to y
The command (set-cdr! x y) changes the value of the cdr position of the cons cell referred by x.
The cdr position is assigned to y
Using the list mutators it is possible to make circular structures
Without use of the list mutators, and with use of structural equivalence predicates, it is not possible to tell the difference
between a list structure and a copy of the list structure
