code: function interpose(coll, separator) {
    if (arguments.length === 1) {
      separator = coll;
      return function(xform) {
        return new Interpose(separator, xform);
      };
    }
    return seq(coll, interpose(separator));
  }
docs: Returns a new collection containing elements of the given
  collection, separated by the specified separator. Returns a
  transducer if a collection is not provided.