R: How to change the text of strips in R's lattice xyplot -

i trying use function histogram plot density of data. sample format of data follows:

   library(lattice)    index<-c(1,1,1,2,2,2,2)    freq<-c(3,4,6,3,6,2,2)    d<-data.frame(index,freq)    histogram(~d$freq|d$index) 

i want have index number printed strip text each histogram (i.e. in example, 1 , 2 on top of histograms respectively instead of printed d$index), don't know of easy way (i know have use strip.custom() possibly, , change var.name properly, don't know how iterate on according index)

if change index variable factor supposed default behavior, i.e. printing shingle levels in strip:

library(lattice)     index<-factor(c(1,1,1,2,2,2,2))     freq<-c(3,4,6,3,6,2,2)     d<-data.frame(index,freq)     histogram(~d$freq|d$index) 

just fun can play style variable in strip.default:

histogram(~freq|index, data=d, strip =        function(..., style){ strip.default(..., style = 4)}  )