title: python recursion combination
body: <p>How can I write a function that computes:</p>
  
  <pre><code>C(n,k)= 1                          if k=0
          0                          if n&lt;k
          C(n-1,k-1)+C(n-1,k)     otherwise
  </code></pre>
  
  <p>So far I have:</p>
  
  <pre><code>def choose(n,k):
  if k==0:
     return 1
  elif n&lt;k:
     return 0
  else:
  </code></pre>