Lab 3:

Develop a representation for fruits; A fruit can be either a lemon, a melon, or
a nomel.  None of these have any fields. You may use a define-type or symbols,
as you like.

Develop the function onlyLemons, that consumes a list of fruits and returns a
list containing only the lemons.

Develop the function onlyMelons, that consumes a list of fruits and returns a
list containing only the melons.

Abstract over the two of these to obtain the function onlyThis, that consumes a
list of fruits and a fruit 'f' and returns a list containing only those elements of
the list that are 'equal?' to 'f'.

Develop the 'my-append' function that consumes two lists and returns the result of
appending the second one to the first.  So, if called with the list [a,b,c] and
the list [d,e,f], it would return [a,b,c,d,e,f].

Develop the 'my-take' function that consumes a list and a number n and returns
the first 'n' elements of the list.  If the list contains fewer than 'n'
elements, it returns the entire list.

Develop the 'my-drop' function that consumes a list and a number n and returns
the list that remains after the first 'n' elements of the input list.  If the
length of the input list is <= n, this function returns an empty list.