This is the use case: abstract Component with a getargs(c::Component) that 
returns an array with the fields of components, for concreteness lets say a 
Der type with the fields x and dy. We want to be able to fetch the fields, 
mutate the array (say replace all :a with :b^2) and then return a mutated 
immutable instance, and we want it defined generally on Component so no 
inner constructors. How?

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