Missing Monoid for Maybe and Add Point

[gallais]

For Maybe: empty is nothing and append is the first-just.

For Add.Point, empty is nothing and append is assuming the set we added a point to is a semigroup.

[lexvanderstoep]

I'm planning to have a go at this issue. Could you explain what the first-just operator refers to?

[gallais]

Maybe is typically used for representing errors. As a consequence the sensible
notion of monoid is one where unit is nothing and multiplication is the function
which returns the first (left to right) value to be a just.

[lexvanderstoep]

If I'm not mistaken, it seems to be that the _<|>_ operator is the first-just operator. The corresponding monoid is <|>-isMonoid in Data.Maybe.Properties.

[jamesmckinna]

So... should this issue now be closed, since #1751 and @lexvanderstoep 's observation above?
Notwithstanding my own concerns under #1957 and the abandoned (for the time being) #1958 and #1963 , which we might revisit in the future...