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Simple transformations for swapping keys and values in clojure maps

Provides four transformation functions:

```
(require '[map-inversions.core :as i])
```

A relation is `one-to-one`

if each key is associated with a unique val.
The result of inverting a one-to-one map is a one-to-one map.

```
user> (i/invert-one-to-one {:a 1 :b 2 :c 3})
{1 :a, 2 :b, 3 :c}
user> (i/invert-one-to-one {:a 1 :b 1 :c 3})
{1 :b, 3 :c}
```

A relation is `many-to-one`

if there are two or more keys that
are associated with the same value - If your values are not unique,
then you probably want this...

```
user> (i/invert-many-to-one {:a 1 :b 2 :c 2})
{1 [:a], 2 [:b :c]}
user> (i/invert-many-to-one {:a 1 :b 2 :c 3})
{1 [:a], 3 [:c] 2 [:b]}
;; note that you can pass an optional item that will serve as
;; the base coll for calls to conj during the construction
;; for performance the default is []
user> (i/invert-many-to-one {:a 1 :b 2 :c 2} #{})
{1 #{:a}, 2 #{:c :b}}
user> (i/invert-many-to-one {:a 1 :b 2 :c 2} nil)
{1 (:a), 2 (:c :b)}
```

A relation is `one-to-many`

if each key is associated with a collection of values
and all collections are disjoint - i.e the union of any two values is empty.

```
user> (i/invert-one-to-many {:a [1 2] :b #{3 4 5} :c (list 6)})
{1 :a, 2 :a, 4 :b, 3 :b, 5 :b, 6 :c}
```

If your values are collections and they contain duplicates, then you just want to use this

```
user> (i/invert-many-to-many {:a [1 2] :b #{3 2 5} :c (list 1 6)})
{1 [:a :c], 6 [:c], 3 [:b], 2 [:a :b], 5 [:b]}
;; again a constructor can be passed
user> (i/invert-many-to-many {:a [1 2] :b #{3 2 5} :c (list 1 6)} nil)
{1 (:c :a), 2 (:b :a), 3 (:b), 5 (:b), 6 (:c)}
user> (i/invert-many-to-many {:a [1 2] :b #{3 2 5} :c (list 1 6)} [:ok])
{1 [:ok :a :c], 2 [:ok :a :b], 3 [:ok :b], 5 [:ok :b], 6 [:ok :c]}
```

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