A variation on the `assert' macro in which the restart handler is bound
during the execution of the test-form, so that uses of this macro can be nested.
As with `assert,' a setf-able `place' may be given, but unlike `assert,' only
one may be given. As with `assert,' you can give an error message, but you can
also give a message to go in the restart handler. That report message may also
use the arguments. It will usually say something like ``supply a new value for
foo.'' The restart is named `continue,' and takes one argument, the value to
setf `place' to. Like `assert,' NIL is returned if test-form is true.