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