new h/w: 
  - findmin (two-argument function)

  - what is the data description and signature
  - more HO signatures
  - list fun applications

  - standard list functions; use them to implement 
      - (make-event time name)
      - findnext, deletenext (simulations)





 7. What general-purpose list-processing functions might match each of
    these signatures?


 7. *Define a function* `vowel?` which tells whether a 1-character
    string is a vowel.  Instead of a complex conditional, find a way
    to use this list:

    > `'(list "a" "e" "i" "o" "u" "y" "A" "E" "I" "O" "U" "Y")`

 1. *Define a function* `has-multiple-vowels?` which is given a string
    and determines if that string has different vowels:

          (check-expect (has-multiple-vowels? "awash") false)
          (check-expect (has-multiple-vowels? "ahoy") true)

    Convert the string to a list, and find a way to use a `local`
    definition with `ormap`. 

 1. *Define a function* that takes a list of strings and returns only
    those strings that don't have different vowels.

 1. *Define a function* that takes a nonempty list of strings and
    returns the string on the list that has the most different
    consonants.

    *Hint*: Remember the examples in class.  Remember the `nub`
    function from the [templates lab](../labs/templates.html).

 1. *Define a function* that takes a nonempty list of integers and returns
    the integer on the list with the most 7's in its decimal
    representation.

    *Hint*: you can use `number->string`.

 1. *Define a function* that takes a nonempty list of images and
    returns the image with the fewest total pixels.

 1. *Define a function* that takes a nonempty list of images and
    returns the image with the shortest long dimension.
    (If an image is in portrait orientation, its long dimension is
    height; if the image is in landscape orientation, its long
    dimension is width.)
    
 1. *Define a function* that takes a mile number and a nonempty list of
    *stations* (from the railway) and returns the station that is
    closest to the given mile marker.

 1. *Define a function* that takes a mile number and a list of
    *stations* (from the railway) and returns all the station that are
    within 50 miles of the given mile marker.