This function deletes a previously defined definstances.
Syntax
(undefinstances <definstances-name>)
If the symbol * is used for <definstances-name>, then all
definstances will be deleted (unless there exists a definstances
called *). The undefinstances command can be used to remove definstances
at any time. Exceptions: A definstances may not be deleted when
any of the instances in it are being created. This function has
no return value.