Editing PNG with private chunks - adding and removing a chunk with public methods

[pnelson-eog]

Right now, the only way to add a chunk is using

push(ancillary chunk:(type:PNG.Chunk, data:[UInt8]),
        pixel:PNG.Format.Pixel, palette:PNG.Palette?,
        background:inout PNG.Background?,
        transparency:inout PNG.Transparency?) throws

For adding the ancillary chunk, the pixel argument should default since it is not used. I have to specify a dummy value that gets ignored.
There isn't a mutating function that allows me to remove or replace the data in a chunk.
Swift's copy on write for value types lets you think you are modifying the metadata.application if you try to remove something but you are only modifying a copy.

[pnelson-eog]

I realize this is complicated by the fact that metadata.application can hold multiple chunks with the same type. Removing a large chunk would be inefficient without some additional identity.