docs(derived): drop the retired 'defined earlier' wording

There is no earlier: dependencies are declared in VAR_DEPS and checked in both
directions, so the rejection is an undeclared reference. Both sentences now say
so.

docs/derived-variables.md

@@ -122,9 +122,9 @@ Derived columns themselves are an EDIT screen feature: the VIEW screen and the d
  - **Read-only.** Derived columns cannot be edited in the grid, and any value submitted for them is discarded before the approval package is created.
  - **Recomputed on every read.** The value always reflects the current data and the current definition, and there is no history of previous derived values.
 
-A configuration error - a bad name, an invalid type or length, a name collision, or a reference to a derived variable that is not defined earlier - stops the EDIT screen with a message naming the variable. A syntax error or an unknown column in the expression itself is caught when the view is compiled, and stops the screen with a message naming the table.
+A configuration error - a bad name, an invalid type or length, a name collision, or a reference to a derived variable that is not declared in `VAR_DEPS` - stops the EDIT screen with a message naming the variable. A syntax error or an unknown column in the expression itself is caught when the view is compiled, and stops the screen with a message naming the table.
 
-Most of these rules are also applied when a row is submitted for approval, through a post edit hook on `MPE_DERIVED`. A bad name, type, length, format or expression - or a reference to a variable that is not defined earlier - is rejected at submit time, while the error can still be corrected, rather than breaking the EDIT screen afterwards. The collision between `VAR_NM` and a physical column of the target table is only checked at read time, as the hook does not have the target library assigned.
+Most of these rules are also applied when a row is submitted for approval, through a post edit hook on `MPE_DERIVED`. A bad name, type, length, format or expression - or a reference to a derived variable that is not declared in `VAR_DEPS` - is rejected at submit time, while the error can still be corrected, rather than breaking the EDIT screen afterwards. The collision between `VAR_NM` and a physical column of the target table is only checked at read time, as the hook does not have the target library assigned.
 
 The simple rules are also configured as validation rules on the table itself, so they show up in the grid before submission: `VAR_TYPE` is a dropdown of `C` and `N`, `VAR_LEN` carries minimum and maximum values, and `VAR_NM`, `VAR_DEPS`, `VAR_FMT` and `VAR_LOGIC` are pattern checked. Validation rules are evaluated in the browser, so the post edit hook remains the authoritative check.