In a nutshell
Provide the (type) language for all the pgn-* packages I am developing.
I have tried to develop parts of the pgn-* packages individually, and to import what is needed. By extracting now pgn-writer (that uses types in part defined by pgn-parser, part by pgn-reader) it gets more and more complicated.
To avoid having duplication of the types (by doing copy and paste), I try now to eliminate duplicates and move them to the common types used in many packages. The size should be minimal (only declaration), but perhaps I gain a common model where the small glitches that are not easy to find can be avoided.
And yes, I will have additional dependencies that has to be managed, with some more changes if types / design changes. But better to have it explicit instead of subtle errors ...