`xor` function in R -

i'm trying work through code in bootstrap simulation on this post 2 major hurdles. first, few times have played bootstrapping, have done without using {boot} package, , relying on functions replicate, or loops. , second, , main object of question, don't know function xor takes in inputs, , does:

n <- 1000 dat <- c("a", rep("b", n-1)) indicator <- function(x, ndx)   xor("a"%in%x[ndx], true)  

i see assesses whether "a" in x , returns false if in it. issue comes input ndx. stands "index", , connected bootstrapping follows:

p_hat <- function(dat, m=1e3){     foo <- boot(data=dat, statistic=indicator, r=m)      1/mean(foo$t) }   reps <- replicate(100, p_hat(dat)) 

but how? in last chunk of code feeds ndx in first chunk?