docs: private static String translateVar(SoyToJsVariableMappings variableMappings, Matcher matcher) {
      Preconditions.checkArgument(matcher.matches());
      String firstPart = matcher.group(1);
  
      StringBuilder exprTextSb = new StringBuilder();
  
      // ------ Translate the first key, which may be a variable or a data key ------
      String translation = getLocalVarTranslation(firstPart, variableMappings);
      if (translation != null) {
        // Case 1: In-scope local var.
        exprTextSb.append(translation);
      } else {
        // Case 2: Data reference.
        exprTextSb.append("opt_data.").append(firstPart);
      }
  
      return exprTextSb.toString();
    }