title: Typescript: Specifying a type for an instance method
body: <p>I'm using Typescript 3.3.
  There is a <code>type</code> from a library: <code>type Fn = (arg0: t1...) =&gt; any</code>,
  and I imported it, want to use it to sign an instance method:</p>
  
  <pre class="lang-js prettyprint-override"><code>class A {
    mtd() {} // to be signed
  }
  </code></pre>
  
  <p>How should I do?</p>