`delta_apply : ('a -> 'a delta) -> 'a -> 'a`
STRUCTURE
SYNOPSIS
Apply a function to an argument, re-using the argument if possible.
DESCRIPTION
An application delta_apply f x applies f to x and, if the result is SAME, returns x. If the result is DIFF y, then y is returned.
FAILURE
If f x raises exception e, then delta_apply f x raises e.
EXAMPLE
Suppose we want to write a function that replaces every even integer in a list of pairs of integers with an odd one. The most basic replacement function is therefore
```   - fun ireplace i = if i mod 2 = 0 then DIFF (i+1) else SAME
```
Applying ireplace to an arbitrary integer would yield an element of the int delta type. It’s not seemingly useful, but it becomes useful when used with similar functions for type operators. Then a delta function for pairs of integers is built by delta_pair ireplace ireplace, and a delta function for a list of pairs of integers is built by applying delta_map.
```   - delta_map (delta_pair ireplace ireplace)
[(1,2), (3,5), (5,7), (4,8)];
> val it = DIFF [(1,3), (3,5), (5,7), (5,9)] : (int * int) list delta

- delta_map (delta_pair ireplace ireplace)
[(1,3), (3,5), (5,7), (7,9)];
> val it = SAME : (int * int) list delta
```
Finally, we can move the result from the delta type to the actual type we are interested in.
```   - delta_apply (delta_map (delta_pair ireplace ireplace))
[(1,2), (3,5), (5,7), (4,8)];
> val it = [(1,3), (3,5), (5,7), (5,9)] : (int * int) list
```