###Extra credit problems###
-<OL start=12>
-<li>In class I mentioned a function `&&` which occupied the position *between* its arguments, rather than coming before them (this is called an "infix" function). The way that it works is that `[1, 2, 3] && [4, 5]` evaluates to `[1, 2, 3, 4, 5]`. Define this function, making use of `letrec` and the simpler infix operation `&`.
+* In class I mentioned a function `&&` which occupied the position *between* its arguments, rather than coming before them (this is called an "infix" function). The way that it works is that `[1, 2, 3] && [4, 5]` evaluates to `[1, 2, 3, 4, 5]`. Define this function, making use of `letrec` and the simpler infix operation `&`.
-<li>Write a function `unmap2` that is something like the inverse of `map2`. This function expects two arguments, the second being a sequence of elements of some type *t*. The first is a function `g` that expects a single argument of type *t* and returns a *pair* of results, rather than just one result. We want to collate these results, the first into one list, and the second into a different list. Then `unmap2` should return those two lists. Thus if:
+* Write a function `unmap2` that is something like the inverse of `map2`. This function expects two arguments, the second being a sequence of elements of some type *t*. The first is a function `g` that expects a single argument of type *t* and returns a *pair* of results, rather than just one result. We want to collate these results, the first into one list, and the second into a different list. Then `unmap2` should return those two lists. Thus if:
- g x1 # evaluates to [y1, z1]
- g x2 # evaluates to [y2, z2]
- g x3 # evaluates to [y3, z3]
+ g z1 # evaluates to [x1, y1]
+ g z2 # evaluates to [x2, y2]
+ g z3 # evaluates to [x3, y3]
-Then `unmap2 (g, [x1, x2, x3])` should evaluate to `([y1, y2, y3], [z1, z2, z3])`.
+ Then `unmap2 (g, [z1, z2, z3])` should evaluate to `([x1, x2, x3], [y1, y2, y3])`.
-<li>Write a function `takewhile` that expects a `p` argument like `filter`, and also a sequence. The result should behave like this:
+* Write a function `takewhile` that expects a `p` argument like `filter`, and also a sequence. The result should behave like this:
- takewhile ((lambda x. x < 10), [1, 2, 20, 4, 40]) # evaluates to [1, 2]
+ takewhile ((lambda x. x < 10), [1, 2, 20, 4, 40]) # evaluates to [1, 2]
-Note that we stop "taking" once we reach `20`, even though there are still later elements in the list that are less than `10`.
+ Note that we stop "taking" once we reach `20`, even though there are still later elements in the list that are less than `10`.
-<li>Write a function `dropwhile` that expects a `p` argument like `filter`, and also a sequence. The result should behave like this:
+* Write a function `dropwhile` that expects a `p` argument like `filter`, and also a sequence. The result should behave like this:
- dropwhile ((lambda x. x < 10), [1, 2, 20, 4, 40]) # evaluates to [20, 4, 40]
+ dropwhile ((lambda x. x < 10), [1, 2, 20, 4, 40]) # evaluates to [20, 4, 40]
-Note that we stop "dropping" once we reach `20`, even though there are still later elements in the list that are less than `10`.
-
-<li>Write a function `reverse` that returns the reverse of a sequence. Thus, `reverse [1, 2, 3, 4]` should evaluate to `[4, 3, 2, 1]`.
-
-</ol>
+ Note that we stop "dropping" once we reach `20`, even though there are still later elements in the list that are less than `10`.
+* Write a function `reverse` that returns the reverse of a sequence. Thus, `reverse [1, 2, 3, 4]` should evaluate to `[4, 3, 2, 1]`.