fdlabels <- function(fdnames, nrep, nvar) {

#  Extract plot labels and, if available, names for each replicate and
#  each variable

#  check fdnames, which must be a list object of length 3

  if (!inherits(fdnames, "list"))
    stop("Argument fdnames is not a list object.")

  if (length(fdnames) != 3)
    stop("Argument fdnames is not of length 3.")

#  xlabel is fdnames[[1]] if it has length 1 and is not null
#  otherwise xlabel is names(fdnames)[1]

  xlabel = fdnames[[1]]
  if (length(xlabel) > 1 || is.null(xlabel)) xlabel = names(fdnames)[1]
  if (!is.character(xlabel)) xlabel = ""

#  ylabel is fdnames[[3]] if it has length not equal to nvar and is not null
#  otherwise ylabel is names(fdnames)[3]

  ylabel = fdnames[[3]]
  if ( (nvar > 1 && length(ylabel) == nvar) || 
      is.null(ylabel)) ylabel = names(fdnames)[3]
  if (length(ylabel) > 1) {
    if     (inherits(ylabel, "character")) ylabel = ylabel[1]
    else {
      if (inherits(ylabel, "list"))        ylabel = ylabel[[1]]
      else                                 ylabel = ""
    }
  }
  if (!is.character(ylabel)) ylabel = ""

#  set up casenames
 
  if (length(fdnames[[2]]) == nrep) {
    casenames = as.character(fdnames[[2]])
  } else {                             
    casenames = NULL
  }

#  set up varnames

  if (length(fdnames[[3]]) == nvar) {
    varnames  = as.character(fdnames[[3]])
  } else {                             
    varnames  = NULL
  }

  return(list(xlabel=xlabel, ylabel=ylabel, 
              casenames=casenames, varnames=varnames)) 
}