Traceback (most recent call last):
  File "/tmp/test.py", line 17, in <module>
    print(rtlil.convert(a))
  File "/home/jf/src/nmigen/nmigen/back/rtlil.py", line 886, in convert
    fragment = fragment.prepare(**kwargs)
  File "/home/jf/src/nmigen/nmigen/hdl/ir.py", line 492, in prepare
    fragment._propagate_ports(ports=(), all_undef_as_ports=True)
  File "/home/jf/src/nmigen/nmigen/hdl/ir.py", line 419, in _propagate_ports
    self._prepare_use_def_graph(parent, level, uses, defs, ios, self)
  File "/home/jf/src/nmigen/nmigen/hdl/ir.py", line 393, in _prepare_use_def_graph
    subfrag._prepare_use_def_graph(parent, level, uses, defs, ios, top)
  File "/home/jf/src/nmigen/nmigen/hdl/ir.py", line 374, in _prepare_use_def_graph
    add_defs(value._lhs_signals())
  File "/home/jf/src/nmigen/nmigen/hdl/ir.py", line 361, in add_defs
    assert defs[sig] is self
AssertionError