let default_stylesheets = [
  "https://cdn.rawgit.com/hammerlab/ketrew/2d1c430cca52caa71e363a765ff8775a6ae14ba9/src/doc/code_style.css";
  "http://cdn.jsdelivr.net/bootstrap/3.1.1/css/bootstrap.min.css";
  "http://cdn.jsdelivr.net/bootstrap/3.1.1/css/bootstrap-theme.min.css";
  (* <link rel="stylesheet" href="code_style.css" type="text/css"> *)
]