title: reducing number of choices
body: <p>I have two tables: </p>
  
  <pre><code>db.define_table('tests', Field('name'), Field('status'),...)
  db.tests.status.requires=IS_IN_SET(['OK','obsolete'])
  db.define_table('testruns', Field('name'), Field('test', db.tests), ...)
  </code></pre>
  
  <p>My form looks lik this:</p>
  
  <pre><code>form = SQLFORM(db.testruns)
  </code></pre>
  
  <p>How can I change the form, that it contains only the tests that have the state 'OK'?</p>