diff --git a/text-crossrefs.lua b/text-crossrefs.lua
index f4a62f3..21b4030 100644
--- a/text-crossrefs.lua
+++ b/text-crossrefs.lua
@@ -205,26 +205,12 @@ local function labelize_span(span)
   end
 end
 
-local function has_class(elem, class)
-  if elem.classes then
-    for i = 1, #elem.classes do
-      if elem.classes[i] == class then
-        return true
-      end
-    end
-    return false
-  else
-    error('function has_class used on an element of type ' ..
-          elem.t .. ' that cannot have classes.')
-  end
-end
-
 local current_note_labels = {}
 
 local collect_note_labels = {
   Span = function(span)
     if span.identifier ~= '' and
-      (config.only_explicit_labels == 'false' or has_class(span, 'label')) 
+      (config.only_explicit_labels == 'false' or span.classes:includes('label')) 
     then
       table.insert(current_note_labels, span.identifier)
     end
@@ -435,7 +421,7 @@ local function format_reference(target, info_type)
 end
 
 local function make_reference(span)
-  if has_class(span, 'ref') then
+  if span.classes:includes('ref') then
     local target = analyze_reference_span(span)
     if not target.is_external then
       local info_type = span.attributes.type or config.default_info_type