Answer product-owner round 1: the Versioning table names the data packages, the README pairs each locale with its geo module, WithDataFS refuses a nil module, errNoData says New was given no data, and three changelog lines the move made false are true again

CHANGELOG.md

@@ -41,8 +41,8 @@ replacement, and each removed path, column or flag.
   where it does nothing; a name nothing reads; a repeated item, which counts as written; a
   weight of `0`, which is never drawn; a `repeat` or count of `0`, which renders nothing;
   `+5`, `05` and `-0`; and a reference into the category it sits in. A rows file no
-  category names is ignored. `New` with no data loads, and renders templates that read
-  none. The CLI's `-n` takes `0`; `--list` ignores `--repeat`, `--separator`, `--format`
+  category names is ignored. `New(WithDataFS())` loads no data, and renders templates that
+  read none. The CLI's `-n` takes `0`; `--list` ignores `--repeat`, `--separator`, `--format`
   and `--table`, and `--table` is ignored without `--format sql`.
 - Table categories: a category JSON naming a `rows` TSV beside it, with the options
   `key`, `name`, `weight` and `parent`; a path selects a row by key or name,
@@ -92,8 +92,8 @@ replacement, and each removed path, column or flag.
   and built from SCB, GeoNames, Trafikverket NVDB and the US Census Bureau, and an
   `address` record binding one locality and drawing its street and postal code inside
   it. `sv_SE.address` and `en_US.address` bind those records once, so
-  `en_US.address.street` no longer carries `name` and `suffix`, and a locale folder
-  loads only beside `geo`.
+  `en_US.address.street` no longer carries `name` and `suffix`, and a locale's `address`
+  reads its country's `geo` module.
 - `{date(from,to,'layout')}` and `{time('layout')}`: a second between two days, or
   within one, in a single-quoted Go layout, drawn in UTC; `from` may equal `to`.
   `sv_SE.date`, `en_US.date`, `sv_SE.time` and `en_US.time` render through them, so
@@ -135,6 +135,6 @@ replacement, and each removed path, column or flag.
 - A data source whose root holds a `.fejkdata.json` manifest with an `index` loads each
   category on the first call reaching it, as each shipped module does. Data failing to load,
   in `New` or on that first call, gives an error matching `ErrLoad`, and the CLI exits 1.
-  A source without an index loads in `New`, with the shipped categories it reads. Every
+  A source without an index loads in `New`, with the indexed categories it reads. Every
   other shipped category loads on the first call reaching it, so one your replacement
   breaks fails there.

README.md

@@ -361,7 +361,8 @@ ok, err := fejkdata.IsTemplate(arg)       // an inline template by its shape, el
 `data.Modules()` is every shipped module, and links all of them into your program. To link
 only the ones you read, pass each package's `FS` with the modules its manifest reads:
 `WithDataFS(sv_SE.FS, SE.FS)` from `github.com/larvit/fejkdata/data/sv_SE` and
-`github.com/larvit/fejkdata/data/geo/SE`. `New` fails where no `WithDataFS` or
+`github.com/larvit/fejkdata/data/geo/SE`, or `en_US.FS` with `US.FS` from
+`github.com/larvit/fejkdata/data/geo/US`; `misc` reads none. `New` fails where no `WithDataFS` or
 `WithDataPath` names the data to load.
 
 `New` refuses data that breaks the grammar, could mean two things or cannot render a valid
@@ -898,6 +899,7 @@ and regenerate seeded fixtures and expected values when you raise that version.
 | Data format | a fence: a spelling a load rejects that it accepted, in a category or a manifest; a template option, since it reserves a field name | a builtin |
 | CLI | remove or rename a flag, or change its default; change what an exit code means; change the framing a `--format` writes (header, quoting, statement shape), the `--list` layout, or what an error names | a flag, a format |
 | Library | change or remove an exported name; raise the lowest supported Go | an exported name, a `With…` option |
+| Data packages | rename or remove a package; move a category to another package; add a module to a package's `reads` | a package |
 
 A patch changes no row of this table: performance, docs, or a fix inside a promised
 behaviour that adds or removes no value and changes no path, format or spelling.

fejkdata.go

@@ -37,6 +37,7 @@ type config struct {
 	seed    uint64
 	seeded  bool
 	sources []datafiles.Source
+	err     error
 	// given is whether an option named the data to load, though it may name none.
 	given bool
 }
@@ -64,14 +65,18 @@ func WithDataPath(dir string) Option {
 // with none loads no data.
 func WithDataFS(modules ...fs.FS) Option {
 	return func(c *config) {
-		for _, fsys := range modules {
+		for i, fsys := range modules {
+			if fsys == nil {
+				c.err = fmt.Errorf("fejkdata: WithDataFS module %d is nil", i+1)
+				continue
+			}
 			c.sources = append(c.sources, datafiles.FS(fsys))
 		}
 		c.given = true
 	}
 }
 
-var errNoData = errors.New("fejkdata: New names no data to load; pass WithDataFS(data.Modules()...), importing github.com/larvit/fejkdata/data, for every module fejkdata ships, WithDataFS or WithDataPath with your own, or WithDataFS() for none")
+var errNoData = errors.New("fejkdata: New was given no data to load; pass WithDataFS(data.Modules()...), importing github.com/larvit/fejkdata/data, for every module fejkdata ships, WithDataFS or WithDataPath with your own, or WithDataFS() for none")
 
 // ErrLoad marks data that fails to load: in New, or on the first call reaching a category
 // of an indexed source.
@@ -99,6 +104,9 @@ func New(opts ...Option) (*Generator, error) {
 	if !c.given {
 		return nil, errNoData
 	}
+	if c.err != nil {
+		return nil, c.err
+	}
 	root, err := loadSources(c.sources)
 	if err != nil {
 		return nil, fmt.Errorf("fejkdata: %w", loadError{err})