Com S 342 --- Principles of Programming Languages
	     LECTURE EXERCISES ON RECURSION ON FLAT LISTS


1. [zip] Write a procedure, zip, that takes two lists and produces a
list of two element lists.  For example,

  (zip '(3 4 2) '(5 9 7))
     ==> ((3 5) (4 9) (2 7))
  (zip '(4 2) '(9 7))
     ==> ((4 9) (2 7))
  
  
  (zip '() '(3 1 4 1 5 9))
     ==> ()
  (zip '(2 7 3 4) '())
     ==> ()













2. [xerox] Write a procedure, xerox, that takes a list of numbers
and returns a list with those numbers duplicated.  For example,

   (xerox '(3 9 2)) ==> (3 3 9 9 2 2)
   (xerox   '(9 2)) ==> (9 9 2 2)
   (xerox      '()) ==> ()














3. [insert-before] Write a procedure, insert-before, that takes two
elements, sought and to-insert, and a list of elements and which
returns a list that is like the argument except that to-insert is
placed in the output list just before the first occurrence of sought.
For example,

   (insert-before 'x 'q '()) ==> ()
   (insert-before 'x 'q '(x y z x))
                   ==> (q x y z x)
   (insert-before 'x 'q '(a x y z))
                   ==> (a q x y z)













4. [insert-before-all] Write a procedure, insert-before, that takes two
elements, sought and to-insert, and a list of elements and which
returns a list that is like the argument except that to-insert is
placed in the output list just before the first occurrence of sought.
For example,

    (insert-before-all 'x 'q '()) ==> ()
    (insert-before-all 'x 'q '(x y z x))
             ==> (q x y z q x)
    (insert-before-all 'x 'q '(a x y z x))
            ==> '(a q x y z q x)