Always greedily `list`-cast`mngrs` input sequence

tractor/trionics/_mngrs.py

@@ -109,6 +109,17 @@ async def gather_contexts(
     all_entered = trio.Event()
     parent_exit = trio.Event()
 
+    # XXX: ensure greedy sequence of manager instances
+    # since a lazy inline generator doesn't seem to work
+    # with `async with` syntax.
+    mngrs = list(mngrs)
+
+    if not mngrs:
+        raise ValueError(
+            'input mngrs is empty?\n'
+            'Did try to use inline generator syntax?'
+        )
+
     async with trio.open_nursery() as n:
         for mngr in mngrs:
             n.start_soon(