diff --git a/src/lumo/data/builder.py b/src/lumo/data/builder.py
index 066e01a..63de7f7 100644
--- a/src/lumo/data/builder.py
+++ b/src/lumo/data/builder.py
@@ -27,16 +27,6 @@ def __init__(self):
 
         self._iter_cache = {}
 
-    def copy(self):
-        db = DatasetBuilder()
-        db._prop = copy.copy(self._prop)
-        db._idx_keys = copy.copy(self._idx_keys)
-        db._data = copy.copy(self._data)
-        db._outs = copy.deepcopy(self._outs)
-        db._transforms = copy.copy(self._transforms)
-        db._outkeys = copy.copy(self._outkeys)
-        return db
-
     def __repr__(self):
 
         if self.sized:
@@ -129,6 +119,23 @@ def _update_len(self):
         self._prop['__clen__'] = res
         return res
 
+    @property
+    def inputs(self):
+        return self._data
+
+    @property
+    def outputs(self):
+        mapping = {}
+        for key, outkeys in self._outs.items():
+            if key == '::idx::':
+                source = range(len(self))
+            else:
+                source = self._data[key]
+
+            for outkey in outkeys:
+                mapping[outkey] = source
+        return mapping
+
     @property
     def mode(self):
         return self._prop.get('mode', 'zip')
@@ -149,6 +156,16 @@ def pseudo_length(self) -> int:
     def pseudo_repeat(self) -> int:
         return self._prop.get('pseudo_repeat', None)
 
+    def copy(self):
+        db = DatasetBuilder()
+        db._prop = copy.copy(self._prop)
+        db._idx_keys = copy.copy(self._idx_keys)
+        db._data = copy.copy(self._data)
+        db._outs = copy.deepcopy(self._outs)
+        db._transforms = copy.copy(self._transforms)
+        db._outkeys = copy.copy(self._outkeys)
+        return db
+
     def subset(self, indices: Sequence[int]):
         self._prop['subindices'] = np.array(indices)
         self._update_len()