Clearly distinguish between a type and the behavioral properties of an instance
of that type. The behavior of a type (and all its subtypes) can now be amended
by adding methods to that types `behavior` object, without exposing the type
objects internals.
E.g.:
pandoc.Inline.behavior.frob = function () print'42' end
local str = pandoc.Str'hello'
str.frob() -- outputs '42'
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