# If the feature FEATURE has not been provided, source it from FILE.
# If FILE is NULL, use the name of FEATURE.

require = function( feature; file )
{
  if ( file == NULL ) { file = feature; }

  if ( !test( feature == $features ) )
  {
    if ( src( file ) != 0 | !test( feature == $features ) )
    {
      message( "run time error: Required feature \"%s\" was not provided.";
               feature );
      exception();
    }
  }
  return feature;
};