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<filter id="InnerShadow">
  <!-- https://gist.github.com/seanbarclay/8627991 -->
  <!-- Shadow Offset, don't make vals lower or you'll break it -->
  <feOffset
    dx='2'
    dy='3'
  />
  <!-- Shadow Blur -->
  <feGaussianBlur
    stdDeviation='1'
    result='offset-blur'
  />
  <!-- Invert the drop shadow to create an inner shadow -->
  <feComposite
    operator='out'
    in='SourceGraphic'
    in2='offset-blur'
    result='inverse'
  />
  <!-- Color & Opacity -->
  <feFlood
    flood-color='black'
    flood-opacity='0.40'
    result='color'
  />
  <!-- Clip color inside shadow -->
  <feComposite
    operator='in'
    in='color'
    in2='inverse'
    result='shadow'
  />
  <!-- Put shadow over original object -->
  <feComposite
    operator='over'
    in='shadow'
    in2='SourceGraphic'
  />
</filter>