walk <- function(currdir,f,arg) {
   # "leave trail of bread crumbs"
   savetop <- getwd()
   setwd(currdir)
   fls <- list.files()
   arg <- f(currdir,fls,arg)
   # subdirectories of this directory
   dirs <- list.dirs(recursive=FALSE)
   for (d in dirs) arg <- walk(d,f,arg)
   setwd(savetop) # go back to calling directory
   arg
}

countinst <- function(startdir,flname) {
   walk(startdir,checkname,
      list(flname=flname,tot=0))
}

checkname <- function(drname,filelist,arg) {
   if (arg$flname %in% filelist)
      arg$tot <- arg$tot + 1
   arg
}

test <- function() {
   unlink('mydir',recursive=TRUE)
   dir.create('mydir')
   file.create('mydir/x')
   dir.create('mydir/d1')
   dir.create('mydir/d2')
   file.create('mydir/d2/x')
   print(countinst('mydir','x'))
}