I understand wanting to have `-Wall -Werror` turned on for development, but I'd rather not have it in the `.cabal` file, since *e.g.* if we upload a version of `diagrams-core` to Hackage that has these options in the `.cabal` file, it will break if some future version of GHC introduces some new warning (when it otherwise might have built just fine).  The general principle is to hold ourselves to strict standards *as developers*, but make things as lax as possible *for users*.