Appender cannot add struct with immutable members

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Wow, very large. I'm going to read this tomorrow in a rested 
state in more detail. But it arouses my interest to write 
something yourself from scratch.
But a quick question: Why is the internal array in the separate 
struct 'Data' encapsulated? To enable a performant postblit, 
since only a pointer need to be copied? If so, I thought the 
nested struct must be static?