wald test for differences

favre <- function(model, coeff.index){

favre2 <- matrix(c(1, -1, 0, 0, 
         1, 0, -1, 0, 
         1, 0, 0, -1
        ), nrow = 3, ncol = 4, byrow = T)

theta.hat <- favre2 %*% as.matrix(model$coeff[coeff.index])
cov.hat <- favre2 %*% vcov(model)[coeff.index, coeff.index] %*% t(favre2)

chisq.stat <- t(theta.hat)%*%solve(cov.hat)%*%theta.hat
  1-pchisq(chisq.stat, df = 3)

}