Syntax::sntxi: Incomplete expression; more input is needed .

ToExpression::esntx: 
   Could not parse (f * g )(t) & \, \stackrel{\mathrm{def}}{=}\
     \int_{-\infty}^\infty f(\tau) g(t - \tau) \, d\tau as input.

<math 
  xmlns='http://www.w3.org/1998/Math/MathML'>
  <semantics>
    <mi>$Failed</mi>
    <annotation-xml encoding='MathML-Content'>
      <ci>$Failed</ci>
    </annotation-xml>
  </semantics>
</math>