A type representing a source element, a target element, or both a source and a target element.
This type represents an inclusive OR relation:
Either a source element is available, or a target element is available, or both are available.
Delta behaves similar to Option, but instead of representing 0 or 1 elements, it represents 1 or 2 elements.
The source and target are described as the two sides of a delta.
Both sides are accessible via optional source and target methods.
Convenient methods like resolve and merge also provide access to the elements.
Transform a Delta value to a different Delta value using map, map_any, or map_all.