Suppose I have a mixed-effects model where yij is the jth sample for  
the ith subject:

yij= beta0 + beta1(age) + beta2(age^2) + beta3(age^3) + beta4(IQ) +   
beta5(IQ^2) + beta6(age*IQ)  + beta7(age^2*IQ)  +  beta8(age^3 *IQ)  
+random intercepti + eij

In R how can I get an F test against the null hypothesis of  
beta6=beta7=beta8=0? In SAS I can run something like contrast  age*IQ  
1, age^2*IQ 1, age^3 *IQ 1, but is there anything similar in R?

Thanks in advance,
Gang
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