## It looks like something like this will meet your needs

## Rich



tmp <- data.frame(trat=letters[1:3], y=rnorm(3), x=1:3)

xyplot(y ~ x | trat, data=tmp)

strip.levelnames <- c(expression(H[2]*O), expression(x^2+y^2),
expression(fraction(x^3, 1+x^3)))

xyplot(y ~ x | trat, data=tmp,
       strip=strip.custom(factor.levels=strip.levelnames))

xyplot(y ~ x | trat, data=tmp,
       strip=strip.custom(factor.levels=strip.levelnames),
       par.strip.text=list(lines=1.5), layout=c(3,1))

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