A somewhat subtle aspect of writing efficient Common Lisp programs is choosing the correct data structures so that the underlying objects can be implemented efficiently. This is partly because of the need for multiple representations for a given value (see section 5.11.2), but is also due to the sheer number of object types that Common Lisp has built in. The number of possible representations complicates the choice of a good representation because semantically similar objects may vary in their efficiency depending on how the program operates on them.