#' Spatial Polygon to Spatial Line
#' @details Polygon to Line.
#'     Funktsioon klassi SpatialPolygons teisendamiseks klassiks SpatialLine.
#' @param sp object with class "SpatialPolygons".
#' @return SpatialLines Object with class "SpatialLines".
#' @seealso \code{\link{isObjectSpatialPolygonsDataFrame}}, \code{\link{centroid2SpatialPolygonsDataFrame}},
#' \code{\link{projection2WGS84}}, \code{\link{area2SpatialPolygonsDataFrame}},
#' \code{\link{perimeter2SpatialPolygonsDataFrame}}, \code{\link{spPolygonsOutline}}
#' @examples \dontrun{
#' load(tr_tsoonid)
#' tr_tsoonid.outline <- perimeter2SpatialPolygonsDataFrame(sp = tr_tsoonid)
#' tr_tsoonid.outline.sl <- spPolygons2SpLines(tr_tsoonid.outline)
#' tr_tsoonid.outline.sl
#' class(tr_tsoonid.outline.sl)
#'
#' }
#'
#' @export
#'

spPolygons2SpLines <- function(sp) {
  methods::as(sp, "SpatialLines")
}