*generate combinations*

If I have 4 flowers to choose from (`#{:rose :lily :daisy :tulip}`), I can generate 4 different combinations of 3 flowers.

```clojure
(#{:rose :lily :daisy}, #{:rose :lily :tulip}, #{:rose :daisy :tulip}, #{:lily
:daisy :tulip})
```

Write a function combinations that takes a collection of values and a number of items to choose and generates all combinations of that size.

Example:

```clojure
(defn combinations [coll n]
 ...)

(combinations #{:rose :lily :daisy :tulip} 3)
; => (#{:rose :lily :daisy}, #{:rose :lily :tulip}, #{:rose :daisy :tulip},
#{:lily :daisy :tulip})
```

Bonus points for clarity, interest, and efficiency.