diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..6e92f57
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+tags
diff --git a/bhl.inc.php b/bhl.inc.php
index 9cc138e..d5b5b5e 100644
--- a/bhl.inc.php
+++ b/bhl.inc.php
@@ -304,3 +304,51 @@ function bhl_line_row_to_pos($file, $line, $row)
   $pos += $row;
   return $pos;
 }
+
+function bhl_map_module_to_file($module)
+{
+  foreach(bhl_proj()->src_dirs as $dir)
+  {
+    $tmp = $dir.'/'.$module.'.bhl';
+    if(file_exists($tmp))
+      return $tmp;
+  }
+  return null;
+}
+
+function bhl_validate_func_ref($module, $func, array $signature)
+{
+  if(sizeof($signature) == 0)
+    throw new Exception("Signature is invalid");
+
+  $module_file = bhl_map_module_to_file($module);
+
+  if(!$module_file)
+    throw new Exception("Module not found '{$module}'");
+
+  $module_src = file_get_contents($module_file);
+
+  $signature_pattern = '';
+  $signature_pattern .= '~func\s+';
+  if($signature[0] !== 'void')
+  {
+    $signature_pattern .= preg_quote($signature[0]);
+    $signature_pattern .= '\s+';
+  }
+  $signature_pattern .= $func;
+  $signature_pattern .= '\s*\(';
+  array_shift($signature);
+  foreach($signature as $type)
+  {
+    $signature_pattern .= '\s*';
+    $signature_pattern .= preg_quote($type);
+    $signature_pattern .= '\s*\S+';
+    $signature_pattern .= '\s*,';
+  }
+  $signature_pattern = rtrim($signature_pattern, '\s*,');
+  $signature_pattern .= '\s*\)';
+  $signature_pattern .= '~';
+
+  if(!preg_match($signature_pattern, $module_src))
+    throw new Exception("Func '$func' signature '".implode(',', $signature)."' not found in module '$module'");
+}
diff --git a/bhl_flt.inc.php b/bhl_flt.inc.php
new file mode 100644
index 0000000..c43f8fd
--- /dev/null
+++ b/bhl_flt.inc.php
@@ -0,0 +1,24 @@
+<?php
+namespace metagen_php;
+use Exception;
+
+function flt_bhl_ref($val, $name, $struct, $args)
+{
+  if(!$val)
+    return $val;
+
+  if(!isset($val['module']))
+    throw new Exception("Missing 'module'");
+
+  if(!isset($val['fn']))
+    throw new Exception("Missing 'fn'");
+
+  $signature = explode(",", $args);
+  if(!$signature)
+    throw new Exception("Invalid signature: $args");
+
+  \taskman\bhl_validate_func_ref($val['module'], $val['fn'], $signature);
+
+  return $val;
+}
+