title: What is wrong in this command "series(diff(f(z), z), z = 1, 2)" in Maple?
body: <p>Assume <code>f(z)</code> is an arbitrary function of <code>z</code>. In Maple, if we want to get the series expansion of <code>diff(f(z),z)</code> at <code>z=0</code>, we can use</p>
  
  <pre><code>series(diff(f(z), z), z = 0, 2)
  </code></pre>
  
  <p>The output is "(D(f))(0)+...". However, we cannot obtain the series expansion of <code>diff(f(z),z)</code> at <code>z=1</code> by</p>
  
  <pre><code>series(diff(f(z), z), z = 1, 2)
  </code></pre>
  
  <p>The output is "Error, (in PDEtools/useD/diff_to_D) invalid input: diff received z+1, which is not valid for its 2nd argument." What is wrong, and how to get the series expansion of an expression that contains derivatives of a function such as <code>diff(f(z),z)</code>?</p>