Plotting shapefile in R: how to delete selected borders?

Question

I'm plotting a shapefile in R and would like to delete certain borders to "group" shapes together. For example, in the plot below, how would I delete the borders between the three blue counties and between the red counties?

library(maptools)

xx <- readShapePoly(system.file("shapes/sids.shp", package="maptools")[1],
 IDvar="FIPSNO", proj4string=CRS("+proj=longlat +ellps=clrk66"))

COLS <- rep(2,100)

COLS[c(9,31,82)] <- 4

plot(xx, axes=TRUE, las=1, col=COLS)

Answer 1

E.g. with gUnaryUnion from the rgeos package:

library("rgeos")
plot(gUnaryUnion(xx), col = "red", axes = TRUE)
plot(gUnaryUnion(xx[c(9, 31, 82), ]), col = "blue", add = TRUE)