Make use of provided assert functions
Passed tests 6.2-6.3:
    "The `cons` function prepends an element to the front of a list."
    "`cons` needs to evaluate its arguments."
Passed test 6.4: "`head` extracts the first element of a list."
Passed test 6.5: "If the list is empty, there is no first element, and
`head` should raise an error."
Passed test 6.6: "`tail` returns the tail of the list."
Passed test 6.7: "The `empty` form checks whether or not a list is
empty."
Passed test 7.2: `or` function
Passed test 7.3: `and` function
Passed test 7.4: `xor` function
Passed test 7.5: `>=` function
Format only
Passed test 7.6: `<=` function
Passed test 7.7: `<` function
Removed unnecessary `if` calls
Passed test 7.8: `sum` function
Passed test 7.9: `length` function
Passed test 7.10: `append` function
Passed test 7.11: `filter` function
Passed test 7.12: `map` function
Passed test 7.13: `reverse` function