title: siir.py unable to run
body: <p>I tried Chris Felton's myHDL <a href="https://bitbucket.org/cfelton/examples/src/6fafd44da12875ad625630328188fea65733278e/siir/siir.py?at=default" rel="nofollow">sample code</a>.</p>
  
  <p>I could not get the Simulation module imported functions to compile with the following errors:</p>
  
  <pre><code>Traceback (most recent call last)
  File "siir.py", line 497, in &lt;module&gt;
  tb = flt.TestFreqResponse(Nloops=3, Nfft=1024)
  File "siir.py", line 408, in TestFreqResponse
  dut = traceSignals(self.RTL, clk, x, y, ts)
  File "C:\WinPython-32bit-2.7.6.3\python-2.7.6\lib\site-   packages\myhdl\_traceSignals.py", line 82, in __call__
  h = _HierExtr(name, dut, *args, **kwargs)
  File "C:\WinPython-32bit-2.7.6.3\python-2.7.6\lib\site- packages\myhdl\_extractHierarchy.py", line 233, in __init__
  _top = dut(*args, **kwargs)
  File "siir.py", line 370, in RTL
  hdl = siir_hdl(clk, x, y, ts, A=self.fxa, B=self.fxb, W=self.W)
  File "siir.py", line 87, in siir_hdl
  assert len(A) == 3, "Tuple of denominator coefficients length 3"
  AssertionError: Tuple of denominator coefficients length 3
  Exception TypeError: 'isinstance() arg 2 must be a class, type, or tuple of classes and   types' in 
  &lt;generator object _LabelGenerator at 0x023ADD00&gt; ignored
  </code></pre>
  
  <p>Could someone kindly advise what I'm doing wrong? Python 2.7 should be ok? Tried it     both as a file and the python interactive command line.</p>