title: how to find word vertically in a crossword
body: <p>I'm trying to write a function that accepts a 2-dimensional (2D) list of characters (like a crossword puzzle) and a string as input arguments, the function must then search the columns of the 2D list to find a match of the word. If a match is found, the function should then return a list containing the row index and column index of the start of the match, otherwise it should return the value None.</p>
  
  <p>For example if the function is called as shown below:</p>
  
  <pre><code>crosswords = [['s','d','o','g'],['c','u','c','m'],['a','c','a','t'],['t','e','t','k']]
  word = 'cat'
  find_word_vertical(crosswords,word)
  </code></pre>
  
  <p>then the function should return:</p>
  
  <pre><code>[1,0]
  </code></pre>