code: def create_index_page(environ, defaults, envdir):
      ''' create the env index html page
      
      Builds the index.html page containing a table of symlinks
      to datamodel directories
  
      Parameters:
          environ (dict):
              A tree environment dictionary
          defaults (dict):
              The defaults dictionary from environ['default']
          envdir (str):
              The filepath for the env directory
      Returns:
          A string defintion of an html page
      '''
  
      # header of index file
      header = 
  
      # footer of index file
      footer = """<h3><a href='{url}/sas/'>{location}</a></h3>
  <p>This directory contains links to the contents of
  environment variables defined by the tree product, version {name}.
  To examine the <em>types</em> of files contained in each environment variable
  directory, visit <a href="/datamodel/files/">the datamodel.</a></p>
  </body></html>
  """
      # create index html file
      index = header.format(**defaults)
      index += create_index_table(environ, envdir)
      index += footer.format(**defaults)
  
      return index
docs: create the env index html page
      
      Builds the index.html page containing a table of symlinks
      to datamodel directories
  
      Parameters:
          environ (dict):
              A tree environment dictionary
          defaults (dict):
              The defaults dictionary from environ['default']
          envdir (str):
              The filepath for the env directory
      Returns:
          A string defintion of an html page