apply allows you to call any procedure, and
specify a list of values to be passed as arguments.
takes a procedure and a list of values, and then calls the procedure
with those values as arguments.
(apply + '(1 2)) passes the values
+, and is equivalent to
(+ 1 2).
You'll seldom need to use
apply, because normal procedure
calling works fine in most situations. Occasionally, though, it is
convenient to be able to apply a procedure to a list of values
that have already been computed. (We'll show an example in [ chapter 4? ].)