A function comparing two object defining equivalence relationship.
In other words if `equal_to` is `EqualDataFunc` then:
`equal_to(a, a)`.
`equal_to(a, b)` then `equal_to(b, a)`.
then `equal_to(a, c)`
The relationship must not change during lifetime of the delegate.
a |
First value |
b |
Second value |
Whether values are equal |