code: function _underline(
      t,              // :: Type
      propPath,       // :: PropPath
      formatType3     // :: Type -> Array String -> String -> String
    ) {
      return formatType3 (t) (propPath) (t.format (_, function(k) {
        return K (_underline (t.types[k],
                              Z.concat (propPath, [k]),
                              formatType3));
      }));
    }
docs: _underline :: ... -> String