error with subtree()

Dear R community,
I would like to plot a partial hclust output, so I?am looking for a subtree function that would return an tree structure I can plot.
I ran the test code of subtree following the instruction on?http://finzi.psych.upenn.edu/library/extracat/html/subtree.html
However, an error message popped up:
====> library(extracat)
Attaching package: ?extracat?
The following object is masked _by_ ?.GlobalEnv?:
? ? subtreehc <- hclust(dist(USArrests), "ave")>?> hcs <- subtree(hc, k = 7)Error in subtree(hc, k = 7) : unused argument (k = 7)
===
Can anyone help me what happened here? And what should I do with it?
Thank you very much!
Ace