Right now I'm working on a lib for charts/plots, and I choose to create a 
custom interval-type that has no comparable requirement.
Of course, as soon as I leave the completely abstract and generic world, I need 
to bring things into an order — but if this order has to be defined by a global 
function "<", there is a loss in flexibility.
My plan is to use "<" as a default wherever possible to define my ranges, with 
the option to change it to a custom alternative; maybe similar reasoning can be 
applied to the index-problem.

Tino

Real world example:
Let's assume I want to create a bar-chart to compare the population of several 
countries.
There is no natural order for countries (I hope we can agree on that ;-), so no 
matter how I'd model them, it would feel wrong to define "<":
Sort by name might be the most obvious choice, but it could also be the size, 
the timezone, average income, the peak hight of its highest mountain...
Additionally, I might want to use different orders for different charts, which 
is only possible when I can change the function which dictates the order.
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