Make everything replaceable goal 9, require changing inputs in the library and default them in the CLI, and record item 77's answers

README.md

@@ -177,9 +177,9 @@ carry sub-fields — `misc.currency.symbol`, `misc.territory.alpha2`,
 
 Where a script under [`data-import/`](data-import) reads a source, the facts come from
 that source, and the script drops a row only by a rule it states; elsewhere a small hand-written
-set ships, and [`todo.md`](todo.md) carries the step that replaces it (goal 13). Copy
+set ships, and [`todo.md`](todo.md) carries the step that replaces it (goal 14). Copy
 that states no fact, such as `word` and `sentence`, stays hand-written. Where a category
-has no path that draws only values reaching nothing real (goal 15), write a fixed value such
+has no path that draws only values reaching nothing real (goal 16), write a fixed value such
 as `example.com` into the template; `todo.md` plans the path.
 
 `misc.httpmethod`, `misc.protocol` and `misc.port` are IANA's registries.
@@ -812,7 +812,7 @@ token {w} is repeated, and uppercase operand "w" holds "w" to one draw per expan
 
 ### Performance
 
-The target is about a microsecond per value (goal 12).
+The target is about a microsecond per value (goal 13).
 
 With only the shipped set, `New` loads nothing: a category loads on the first `Fake`,
 `FakeRecord`, `FakeStruct` or template that reads it, with every category it reads and
@@ -889,6 +889,8 @@ Where the project is heading; the sections before Audience document what ships t
    1. It covers the ten most-spoken languages and the Nordic countries.
    2. Each locale covers the common categories: names, addresses, phone numbers, ids,
       companies and dates.
+   3. A library gets only the data and functions it hands to `New`, and fejkdata's own
+      load the same way as anyone else's.
 3. **The code is easy to understand.**
    1. You can go from a bug to the code behind it without asking anyone.
    2. You can change one part without keeping the rest in your head.
@@ -916,23 +918,42 @@ Where the project is heading; the sections before Audience document what ships t
    wrong.**
    1. A template that could render a value its own rules forbid, such as `NaN` or `Inf`,
       fails to load instead.
+   2. Data that reads something nobody provided fails to load, and the error names the
+      package that provides it by default.
 7. **One command gets you a value.**
    1. Flags work like in other command-line tools, and can go anywhere on the line.
    2. Your first template of your own needs no escaping and no options.
+   3. The CLI fills in what you leave out with a sane default: all the shipped data, the
+      system's randomness and today's date.
 8. **Data is JSON files and TSV tables. Code is only for what they can't express.**
-9. **The same seed, version and data always give the same output.**
-10. **It depends on nothing but Go's standard library.**
-11. **The docs are a map of the template language.**
+9. **Everything is replaceable.** Data, functions, and the packages they depend on can
+   come from fejkdata or anyone else.
+   1. A package names the packages it reads by default, and anything providing the same
+      paths can stand in for one.
+   2. Loading two things under one name fails, in the library and the CLI alike, unless
+      you say the second replaces the first.
+   3. The template language's own functions stay in the core, so a template means the
+      same in every program.
+   4. A function loaded from a package can ask for the input it needs, and gets the
+      randomness and the date the generator uses.
+   5. A CLI carrying your own packages is a few lines of Go, and works just like
+      `fejkdata`.
+10. **The same seed, version and data always give the same output.**
+    1. A library call takes everything that can change its output from its caller: data,
+       randomness and the date.
+11. **It depends on nothing but Go's standard library.**
+12. **The docs are a map of the template language.**
     1. Every feature has its own heading.
     2. Every example runs as a test and shows its output.
     3. Each rule is stated in one place.
-12. **It is fast enough that you never notice it.**
-13. **Every fact comes from an official or open source,** such as a government
+13. **It is fast enough that you never notice with the shipped data, and its cost grows
+    with the data a render reaches, not with all the data loaded.**
+14. **Every fact comes from an official or open source,** such as a government
     register.
     1. A table holds every row its source holds, and nothing added by hand.
-14. **The most common data comes first,** such as names, addresses, phone numbers and
+15. **The most common data comes first,** such as names, addresses, phone numbers and
     dates.
-15. **Values look real by default. You can also ask for values that can never reach
+16. **Values look real by default. You can also ask for values that can never reach
     anything real,** such as a domain nobody can register.
 
 ## Layout