title: Scipy - signaltools.lfilter() function implementation
body: <p>I'm working on a butterworth-filter in java. Therefore I've been looking in scipys implementation.</p>
  <p>Designing the filter went pretty well, but now I'm stuck. I'm trying to implement a filtfilt() function which uses the lfilter() from the title. This method returns</p>
  <pre><code>sigtools._linear_filter(b, a, x, axis)
  </code></pre>
  <p>So now to my actual question:</p>
  <p>Is there any way to have an insight in this _linear_filter() function or can anyone tell me where to find it?</p>