As always, please include a contract, a purpose, examples, and tests for each function.

Exercise 9.5.3 in Section 9.5. Note that the last line asks you to generalize the function. The generalized function should work as follows:

```
(price-below-threshold? (cons .75 (cons 1.95 (cons .25 empty))) 1.5)
;; expected value
false
(price-below-threshold? (cons .75 (cons 1.95 (cons .25 empty))) 2.0)
;; expected value
true
```

Exercise 9.5.4 in Section 9.5 Note that the exercise also asks you to generalize a function to a function that takes three parameters: a list of temperatures, a lower bound, and an upper bound. Assume that the interval is inclusive, i.e. the bounds represent acceptable temperatures. For instance:

```
(check-range? (cons 36.6 (cons 0.0 (cons 75 empty))) 5 95)
;; expected value
false
(check-range? (cons 36.6 (cons 0.0 (cons 75 empty))) 0.0 95)
;; expected value
true
```

Write a function `double`

that takes a list of numbers and
produces a new list in which every number has been multiplied by
2. For instance:

```
(double (list 2 3 7))
;; expected value
(list 4 6 14)
```

Write a function `sum-lists`

that takes two lists of
numbers and creates a new
list in which the first element is the sum of the first elements of
the two lists, the second element is the sum of the two second
elements, etc. If one list is shorter than the other then zero is
added to that element. For instance:

```
(sum-lists (list 1 2) (list 3 4))
;; expected value
(list 4 6)
(sum-lists (list 2 3) (list 5 6 7))
;; expected value
(list 7 9 7)
(sum-lists empty (list 5))
;; expected value
(list 5)
```

Exercise 10.2.4 in Section
10.2. A phone record is a structure that has a name (a symbol)
and a phone number (a 7-digit integer, for simplicity assuming the
same area code for everyone). A phone directory is a list of phone
records.

If a phone or a name is not in a phone directory, a symbol 'NotFound
is returned.

**Update** Below is an example of a phone directory and the
use of the two functions:

```
(define phone-directory (list
(make-phone-entry 'SallySmith 3456789)
(make-phone-entry 'BobAnderson 1234567)
))
(whose-number 3456789 phone-directory)
;; expected value
'SallySmith
(whose-number 4444222 phone-directory)
;; expected value
'NotFound
(phone-number 'SallySmith phone-directory)
;; expected value
3456789
(phone-number 'MarySmith phone-directory)
;; expected value
'NotFound
```

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