code: function(prefix)
  {
     var count = -1;
     var ng = {
        next: function()
        {
           ++count;
           return ng.current();
        },
        current: function()
        {
           return '_:' + prefix + count;
        },
        inNamespace: function(iri)
        {
           return iri.indexOf('_:' + prefix) === 0;
        }
     };
     return ng;
  }
docs: Creates a blank node name generator using the given prefix for the
  blank nodes.
  
  @param prefix the prefix to use.
  
  @return the blank node name generator.