docs(config): optional VAR_LABEL

docs/derived-variables.md

@@ -24,6 +24,7 @@ Each row defines one variable:
  - `VAR_TYPE` - `C` for character, `N` for numeric
  - `VAR_LEN` - character length, or numeric length (1-8)
  - `VAR_FMT` - optional, eg `DATE9.` or `DOLLAR12.2`
+ - `VAR_LABEL` - optional; when blank the label is `<VAR_NM>=<expression>`
  - `VAR_LOGIC` - the SAS expression that produces the value
  - `ACTIVE` - set to 1 to apply the variable
 
@@ -35,6 +36,7 @@ VAR_DEPS    STATE
 VAR_TYPE    C
 VAR_LEN     8
 VAR_FMT     (blank)
+VAR_LABEL   (blank)
 VAR_LOGIC   ifc(STATE in ('CA','OR','WA'),'West','East')
 ACTIVE      1
 ```
@@ -47,6 +49,7 @@ VAR_DEPS    REGION STATE
 VAR_TYPE    C
 VAR_LEN     14
 VAR_FMT     (blank)
+VAR_LABEL   Region and state
 VAR_LOGIC   cats(REGION,'-',STATE)
 ACTIVE      1
 ```
@@ -60,7 +63,7 @@ attrib REGION length=$8 label='REGION=ifc(STATE in (''CA'',''OR'',''WA''),''West
 REGION = ifc(STATE in ('CA','OR','WA'),'West','East');
 ```
 
-The label is the variable name, an equals sign and the expression - `REGION=ifc(...)` - so the column shows both what it is and where it comes from when labels are switched on. The expression is truncated to the maximum label length.
+The label is `VAR_LABEL` when one is supplied, otherwise the variable name, an equals sign and the expression - `REGION=ifc(...)` - so the column shows both what it is and where it comes from when labels are switched on. Either way the label is truncated to the maximum label length, on a boundary that cannot split a doubled quote.
 
 The type and length are declared, not inferred, so a value is truncated exactly as you specify. If `VAR_FMT` is left blank, character variables default to `$<length>.` and numeric variables to `<length>.`; a date, time or datetime format makes the column render (and filter) as a date, time or datetime, the same way a stored column does.