title: How do I tell sympy that ∂b[a]/∂b==1? Derivative on IndexedBase
body: <p>I have an expression which includes an <code>IndexedBase</code>.  I'm taking a partial derivative, but in the output, this is retained:</p>
  
  <pre><code>In [105]: sympy.IndexedBase(b)[a].diff(b)
  Out[105]: 
  ∂       
  ──(b[a])
  ∂b      
  </code></pre>
  
  <p>My <code>b[a]</code> is simply an array <code>b_0</code>, <code>b_1</code>, ..., <code>b_n</code>.  Those are all constants, so ∂b[a]/∂b is equal to 1.  How do I rephrase my problem such that it evaluates to one?</p>