#' @title Partial ranking as directed graph

#' @description Turns a partial ranking into a directed graph. An edge (u,v) is 

#' present if `P[u,v]=1`, meaning that u is dominated by v.

#'

#' @param P A partial ranking as matrix object calculated with [neighborhood_inclusion]

#'    or [positional_dominance].

#' @return Directed graph as an igraph object.

#' @author David Schoch

#' @examples

#' library(igraph)

#' g <- threshold_graph(20,0.1)

#' P <- neighborhood_inclusion(g)

#' d <- dominance_graph(P)

#' \dontrun{plot(d)}

#' 

#' # to reduce overplotting use transitive reduction

#' P <- transitive_reduction(P)

#' d <- dominance_graph(P)

#' \dontrun{plot(d)}

#' @export

dominance_graph <- function(P) {

}