[R] Subsetting problem

[Cressoni, Massimo (NIH/NHLBI) [F]]

I need to perform the Exact Wilcoxon Mann-Whitney on a subset of my database.
Assuming that IPPO is my data frame and IPPOBIS is the subset my variable still
have 3 different levels and the function wilcox_test (package "coin")
does not accept it. 
I do not know how to overcome this problem.

ippo <- c(rep("A",10),rep("B",10),rep("C",10))
ippo2 <- c(rnorm(10,0,1),rnorm(10,10,10),rnorm(10,10,10))
IPPO <- data.frame(ippo,ippo2)

IPPOBIS <- IPPO[IPPO$ippo == "A" | IPPO$ippo == "B",]

wilcox_test(ippo2 ~ ippo,data=IPPOBIS,distribution=exact())
Error in check(itp) : 'object' does not represent a two sample problem
levels(IPPOBIS$ippo)
[1] "A" "B" "C"

Massimo Cressoni