which.site <- function(di, dist, print=T) {
  # given integers of observations in a distance object
  # identify the pair of sites involved

  # input:
  #  di: scalar or vector of integers, e.g. from identify
  #  dist: distance object

  if ((attr(dist,'Diag') != F) & (attr(dist,'Upper') != F)) {
     stop('Distance matrix must be lower triangular without a diagonal') }

  nsamp  <- attr(dist,'Size')
 
  ncol <- rep(1:(nsamp-1), seq(nsamp-1,1,-1))
  nrow <- NULL
  for (i in 2:nsamp) {
   nrow <- c(nrow,i:nsamp)
   }

  sitenames <- attr(dist,'Labels')

  temp2 <- cbind(Row=sitenames[nrow[di]],Col=sitenames[ncol[di]])
  temp <- cbind(Row=nrow[di],Col=ncol[di])
  dimnames(temp)[[1]] <- dimnames(temp)[[1]] <- di

  if (print) {print(temp2, quote=F) }
  invisible(temp)
  }