diff --git a/filter.lua b/filter.lua
index 492096e..4abbaa7 100644
--- a/filter.lua
+++ b/filter.lua
@@ -13,6 +13,57 @@ local function add_edge(u, v)
     table.insert(adj[u], v)
 end
 
+---------------------------------------------------------
+-- Topological sort (DFS-based, with cycle detection)
+---------------------------------------------------------
+local visited = {}  -- "perm" mark: node completely processed
+local in_stack = {} -- "temp" mark: node currently in recursion stack
+local order = {}    -- reverse topological order
+local cycle = nil   -- store cycle if found
+
+local function _dfs(u, path)
+    visited[u] = true
+    in_stack[u] = true
+    table.insert(path, u)
+
+    for _, v in ipairs(adj[u] or {}) do
+        if not visited[v] then
+            _dfs(v, path)
+            if cycle then return end
+        elseif in_stack[v] then
+            -- found a cycle
+            cycle = {}
+            -- extract the cycle part from path
+            for i = #path, 1, -1 do
+                table.insert(cycle, 1, path[i])
+                if path[i] == v then break end
+            end
+            table.insert(cycle, v) -- close the cycle
+            return
+        end
+    end
+
+    in_stack[u] = false
+    table.remove(path)
+    table.insert(order, 1, u)
+end
+
+local function topo_sort()
+    for u, _ in pairs(adj) do
+        if not visited[u] then
+            _dfs(u, {})
+            if cycle then return nil, cycle end
+        end
+    end
+
+    -- build rank map
+    local rank = {}
+    for i, u in ipairs(order) do
+        rank[u] = i
+    end
+    return rank
+end
+
 local function collect_labels(blk)
     if blk.identifier and blk.identifier ~= "" then
         label_map[blk.identifier] = blk:clone()
@@ -100,6 +151,7 @@ return {
             dfs(blk, {})
         end
 
+        -- sanity check
         show("edges:")
         for u, vs in pairs(adj) do
             for _, v in ipairs(vs) do
@@ -107,10 +159,22 @@ return {
             end
         end
 
-        -- replace
-        for i, blk in ipairs(doc.blocks) do
-            doc.blocks[i] = replace(blk)
+        -- topological sort
+        local rank, cycle = topo_sort()
+        if cycle then
+            error("Cycle detected:" .. table.concat(cycle, " -> ") .. "\n")
         end
+
+        -- sanity check
+        show("ranks:")
+        for k, v in pairs(rank) do
+            show(k .. "--" .. v)
+        end
+
+        -- replace
+        -- for i, blk in ipairs(doc.blocks) do
+        --     doc.blocks[i] = replace(blk)
+        -- end
         return doc
     end
 }
diff --git a/output.html b/output.html
index cbdddac..308a0a0 100644
--- a/output.html
+++ b/output.html
@@ -4,12 +4,7 @@
 <p>thm2 - inthm2 (need thm1)</p>
 </div>
 <div id="inc" data-include="thm2 , fakelabel">
-<div id="thm2" class="Theorem">
-<p>test thm2</p>
-<div id="inthm2" data-include="thm1">
-<p>i need theorem 1</p>
-</div>
-</div>
+<p>This line will be ignored</p>
 </div>
 <div id="thm1" class="Theorem">
 <p>test thm1</p>
diff --git a/test.lua b/test.lua
new file mode 100644
index 0000000..ad0de98
--- /dev/null
+++ b/test.lua
@@ -0,0 +1,79 @@
+-- adjacency list: adj[u] = {v1, v2, ...}
+local adj = {}
+
+local function add_edge(u, v)
+    adj[u] = adj[u] or {}
+    table.insert(adj[u], v)
+end
+
+-- Example graph
+add_edge("A", "B")
+add_edge("B", "C")
+add_edge("C", "D")
+-- Uncomment next line to introduce a cycle:
+add_edge("A", "D")
+
+---------------------------------------------------------
+-- Topological sort (DFS-based, with cycle detection)
+---------------------------------------------------------
+local visited = {}  -- "perm" mark: node completely processed
+local in_stack = {} -- "temp" mark: node currently in recursion stack
+local order = {}    -- reverse topological order
+local cycle = nil   -- store cycle if found
+
+local function dfs(u, path)
+    visited[u] = true
+    in_stack[u] = true
+    table.insert(path, u)
+
+    for _, v in ipairs(adj[u] or {}) do
+        if not visited[v] then
+            dfs(v, path)
+            if cycle then return end
+        elseif in_stack[v] then
+            -- found a cycle
+            cycle = {}
+            -- extract the cycle part from path
+            for i = #path, 1, -1 do
+                table.insert(cycle, 1, path[i])
+                if path[i] == v then break end
+            end
+            table.insert(cycle, v) -- close the cycle
+            return
+        end
+    end
+
+    in_stack[u] = false
+    table.remove(path)
+    table.insert(order, 1, u)
+end
+
+local function topo_sort()
+    for u, _ in pairs(adj) do
+        if not visited[u] then
+            dfs(u, {})
+            if cycle then return nil, cycle end
+        end
+    end
+
+    -- build rank map
+    local rank = {}
+    for i, u in ipairs(order) do
+        rank[u] = i
+    end
+    return rank
+end
+
+---------------------------------------------------------
+-- Run it
+---------------------------------------------------------
+local rank, cycle = topo_sort()
+if cycle then
+    print("Cycle detected:")
+    print(table.concat(cycle, " -> "))
+else
+    print("Topological order ranks:")
+    for k, v in pairs(rank) do
+        print(k, v)
+    end
+end