title: python-constraint add constraint
body: <p>I'm developing a python program to solve constraint satisfaction problem(CSP).</p>
  
  <p>Here I have variables list <strong>['MI428', 'UL867', 'QR664', 'TK730', 'UL303']</strong> and their possible assignments <strong>['A1', 'A2', 'B1', 'B2', 'C1']</strong>.</p>
  
  <p>My constraint for this is, having compatibility list for each element in 2nd list (possible assignments). It works as follows;</p>
  
  <p>From the elements of first list(variables list) there is another index to get another key. From that key, I can access set of compatible values(possible assignment) in the list. To better understand consider this example:</p>
  
  <blockquote>
    <p>For variable <strong>'MI428'</strong> I have the key <strong>'B320'</strong> <strong>('MI428->'B320')</strong>, Then I have List of compatible values for <strong>B320</strong> as <strong>['A1', 'A2']</strong></p>
  </blockquote>
  
  <p>Here my task is to assign compatible value from <strong>['A1', 'A2', 'B1', 'B2', 'C1']</strong> to each of variables such as <strong>'MI428'</strong>, satisfying the above explained constraint.</p>
  
  <p>Note: for this I'm using <a href="https://labix.org/python-constraint" rel="nofollow noreferrer">python-constraint</a> Library. I need implementation using that. So far I created a problem instant as follows, so what I really want is adding constraint for that problem.</p>
  
  <pre><code>from constraint import Problem
  problem = Problem()
  problem.addVariables(['MI428', 'UL867', 'QR664', 'TK730', 'UL303'], ['A1', 'A2', 'B1', 'B2', 'C1'])
  problem.addConstraint() // I need this line to be implemented
  solutions = problem.getSolutions()
  </code></pre>
  
  <p>I need a proper constraint for addConstraint line..</p>