diff --git a/masque/library.py b/masque/library.py
index f25c032..6e84d77 100644
--- a/masque/library.py
+++ b/masque/library.py
@@ -841,7 +841,7 @@ class LazyLibrary(MutableLibrary):
     This class is usually used to create a library of Patterns by mapping names to
      functions which generate or load the relevant `Pattern` object as-needed.
     """
-    dict: Dict[str, Callable[[], 'Pattern']]
+    dict: Dict[str, Union[Callable[[], 'Pattern'], Callable[[], 'Tree']]]
     cache: Dict[str, 'Pattern']
     _lookups_in_progress: Set[str]
 
@@ -853,7 +853,7 @@ class LazyLibrary(MutableLibrary):
     def __setitem__(
             self,
             key: str,
-            value: Union['Pattern', Callable[[], 'Pattern']],
+            value: Union['Pattern', Callable[[], 'Pattern'], Callable[[], 'Tree']],
             ) -> None:
         if key in self.dict:
             raise LibraryError(f'"{key}" already exists in the library. Overwriting is not allowed!')
@@ -887,8 +887,14 @@ class LazyLibrary(MutableLibrary):
 
         self._lookups_in_progress.add(key)
         func = self.dict[key]
-        pat = func()
+        pat_or_tree = func()
         self._lookups_in_progress.remove(key)
+        if isinstance(pat_or_tree, Tree):
+            tree = pat_or_tree
+            pat = tree[tree.top]
+            del tree[tree.top]
+            self.add(tree)
+
         self.cache[key] = pat
         return pat