Feature request: [D] as notation for class of D

[mahrud]

Very frequently in math, given an object (say divisor) D and an equivalence relation D~D', we denote the equivalency class with [D]. I think it would be cool if we could define methods on [T], for instance:

[Divisor] := D -> -* the class of D in the class group *-

How hard would this be?

Related: #1978

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As a potentially far fetched extension, allowing (a,b] and [a,b) to produce half open intervals would be useful (both [a,b] and (a,b) work fine and can be interpreted appropriately by methods, so that's not a concern).