move bake-cake to after add
Exercise 6

Write a function squeezed? which returns
true if the given ingredient (the argument)
needs squeezing. Otherwise it returns false.
Exercise 7

Write a function simple? which returns true
if the given ingredient (the argument) can be
added without scooping or squeezing
(basically, butter). Otherwise it returns false.
Exercise 8

Write functions add-scooped, add-squeezed,
and add-simple which
conditionally add the respective ingredient
types.
remove obsolete functions
Exercise 9

Write the following functions using dotimes:

add-flour-cups
add-sugar-cups
add-milk-cups
add-butters