fhdl/structure: clarify usage restrictions of LHS Cat

diff --git a/migen/fhdl/structure.py b/migen/fhdl/structure.py
index 688f2468c758a8e6f4f4ff8cd708d2bfa5bd1844..a94c198b643a0abcaf1fbed754b83a62f601e1b8 100644 (file)
--- a/migen/fhdl/structure.py
+++ b/migen/fhdl/structure.py
@@ -151,7 +151,8 @@ class Cat(Value):
        The return value can be used on either side of an assignment, that
        is, the concatenated value can be used as an argument on the RHS or
        as a target on the LHS. If it is used on the LHS, it must solely
-       consist of `Signal` s. The bit length of the return value is the sum of
+       consist of `Signal` s, slices of `Signal` s, and other concatenations
+       meeting these properties. The bit length of the return value is the sum of
        the bit lengths of the arguments::
 
                flen(Cat(*args)) == sum(flen(arg) for arg in args)