docs: private String valueArgumentFromCompatibleWithAnnotation(AnnotationTree tree) {
      ExpressionTree argumentValue = Iterables.getOnlyElement(tree.getArguments());
      if (argumentValue.getKind() != Kind.ASSIGNMENT) {
        // :-| Annotation symbol broken. Punt?
        return null;
      }
  
      return ASTHelpers.constValue(((AssignmentTree) argumentValue).getExpression(), String.class);
    }