--- a/hgext3rd/evolve/exthelper.py	Sun Dec 02 16:48:05 2018 +0100
+++ b/hgext3rd/evolve/exthelper.py	Fri Nov 30 22:10:39 2018 -0500
@@ -13,6 +13,7 @@
     commands,
     configitems,
     extensions,
+    fileset,
     registrar,
     revset,
     templatekw,
@@ -33,6 +34,7 @@
         self._extcallables = []
         self._repocallables = []
         self._revsetsymbols = []
+        self._filesetsymbols = []
         self._templatekws = []
         self._commandwrappers = []
         self._extcommandwrappers = []
@@ -63,6 +65,7 @@
         self._extcallables.extend(other._extcallables)
         self._repocallables.extend(other._repocallables)
         self._revsetsymbols.extend(other._revsetsymbols)
+        self._filesetsymbols.extend(other._filesetsymbols)
         self._templatekws.extend(other._templatekws)
         self._commandwrappers.extend(other._commandwrappers)
         self._extcommandwrappers.extend(other._extcommandwrappers)
@@ -135,6 +138,12 @@
             revsetpredicate(name)(symbol)
         revset.loadpredicate(ui, 'evolve', revsetpredicate)
 
+        filesetpredicate = registrar.filesetpredicate()
+        for name, symbol in self._filesetsymbols:
+            filesetpredicate(name)(symbol)
+        # TODO: Figure out the calling extension name
+        fileset.loadpredicate(ui, 'exthelper', filesetpredicate)
+
         templatekeyword = registrar.templatekeyword()
         for name, kw, requires in self._templatekws:
             if requires is not None:
@@ -238,6 +247,23 @@
             return symbol
         return dec
 
+    def fileset(self, symbolname):
+        """Decorated function is a fileset symbol
+
+        The name of the symbol must be given as the decorator argument.
+        The symbol is added during `extsetup`.
+
+        example::
+
+            @eh.fileset('lfs()')
+            def filesetbabar(mctx, x):
+                return mctx.predicate(...)
+        """
+        def dec(symbol):
+            self._filesetsymbols.append((symbolname, symbol))
+            return symbol
+        return dec
+
     def templatekw(self, keywordname, requires=None):
         """Decorated function is a template keyword