Retire the public-API doc and its dead analyzer settings

AGENTS.md

@@ -367,9 +367,11 @@ are short, and a stale one is worse than none — if you change what a file desc
 | [`WhatsNew/version_performance_control.md`](Sources/AngouriMath/Docs/WhatsNew/version_performance_control.md) | the inter-version performance table, and how to add a column |
 | `Sources/Analyzers/` | the custom analyzers, including the static-field one behind `[ConstantField]` |
 
-`Docs/Contributing/RS1617Errors.md` is the one exception: it describes adding public members to a
-`PublicApi.*.txt`, and neither those files nor the analyzer that wanted them are in the tree any
-more. Do not follow it; delete or rewrite it if you are in there anyway.
+Anything added for the library's own purposes is not `public` — see
+[`Contributing/coding_rules.md`](Sources/AngouriMath/Docs/Contributing/coding_rules.md). Nothing
+checks that any more: the `PublicApiAnalyzers` package that required every public member to be
+listed in a `PublicApi.*.txt` is gone from the tree, so it is a rule to follow rather than one to
+be caught by.
 
 ## Where the work is
 

Sources/.editorconfig

@@ -3,12 +3,6 @@
 # CS0660: 'IntervalPiece' defines operator == or operator != but does not override Object.Equals(object o)
 dotnet_diagnostic.CS0660.severity = suggestion
 
-# RS0016: some public method is not mentioned as public API
-dotnet_diagnostic.RS0016.severity = error
-
-# RS0017: some method mentioned as public API is not public
-dotnet_diagnostic.RS0017.severity = error
-
 # Missing header warning
 dotnet_diagnostic.IDE0073.severity = warning
 

Sources/AngouriMath/Docs/Contributing/README.md

@@ -12,8 +12,8 @@ If you aren't sure about what to add, you may want to check the current projects
 3. <a href="./ImproveParser.md">Improve parser</a>
 4. <a href="./Transformations.md">Transformations</a> — the layer the 1.x entry points sit on, and
    how to add the next rule set or transformation
-5. <a href="./RS1617Errors.md">Adding a public member</a> — out of date; the `PublicApi.*.txt` files
-   and the analyzer that required them are no longer in the tree
+5. <a href="./coding_rules.md">Coding rules</a> — sealed-or-abstract, immutability, and what may be
+   made `public`
 
 See also <a href="../../../../BREAKING-CHANGES.md">BREAKING-CHANGES.md</a>, where a change that makes
 the same input give a different answer is recorded, and

Sources/AngouriMath/Docs/Contributing/coding_rules.md

@@ -8,4 +8,16 @@ Each inheritable type is either abstract or sealed.
 
 ### Immutability
 
-It should be guaranteed that the user cannot change fields of a record which is inherited from `Entity`. 
+It should be guaranteed that the user cannot change fields of a record which is inherited from `Entity`.
+
+### Visibility
+
+Anything added for the library's own purposes is not `public`. A type used internally stays
+`internal`; a method added to a `public` type stays `internal` or `private` unless it is meant for
+callers. `public` is a promise that
+[BREAKING-CHANGES.md](../../../../BREAKING-CHANGES.md) then has to keep, so it is worth making
+deliberately rather than by default.
+
+This used to be enforced by the `PublicApiAnalyzers` package, which required every public member to
+be listed in a `PublicApi.*.txt`. Neither the package nor those files are in the tree any more, so
+nothing checks it — which makes it a rule to follow rather than one to be caught by.