arrows on contour lines

Hello everybody

I'm using contour() to draw streamlines of potential flow, eg

  jj <- seq(from= -4, to=4,len=20)
  jj <- outer(jj,jj,function(x,y){x})+1i*outer(jj,jj,function(x,y){y})

  f <- function(x){x^2}
  contour(Im(f(jj)), nlevels=44 , labels="")

vec<-rep(c(1,2),22)
set.seed(1)
vec<-sample(vec,44)
  contour(Im(f(jj)), nlevels=44 , labels=c(">","<")[vec])

How best to put arrows on the contour lines to show the direction of
flow?
  (ie  I want contour lines looking like ---<---  and ---->----)

I think this is a lot of work, which I don't mind doing, but it'd be
good to hear the list's ideas before starting to bark up the wrong
tree, so to speak.



-- 
Robin Hankin
Uncertainty Analyst
Southampton Oceanography Centre
SO14 3ZH
tel +44(0)23-8059-7743
initialDOTsurname at soc.soton.ac.uk (edit in obvious way; spam
precaution)