Add instance __ua_convert__ wrapper.

uarray/_backend.py

@@ -511,3 +511,28 @@ def __ua_convert__(dispatchables, coerce):
         return converted
 
     return __ua_convert__
+
+
+def wrap_single_convertor_instance(convert_single):
+    """
+    Wraps a ``__ua_convert__`` defined for a single element to all elements.
+    If any of them return ``NotImplemented``, the operation is assumed to be
+    undefined.
+
+    Accepts a signature of (value, type, coerce).
+    """
+
+    @functools.wraps(convert_single)
+    def __ua_convert__(self, dispatchables, coerce):
+        converted = []
+        for d in dispatchables:
+            c = convert_single(self, d.value, d.type, coerce and d.coercible)
+
+            if c is NotImplemented:
+                return NotImplemented
+
+            converted.append(c)
+
+        return converted
+
+    return __ua_convert__