```
z1 <- rnorm(4000)
z2 <- rnorm(length(z1))
u <- rnorm(length(z1))
# make x1, x2 correlated with errors u
x1 <- z1 + z2 + 0.2*u + rnorm(length(z1))
x2 <- z1 + 0.94*z2 - 0.3*u + rnorm(length(z1))
y <- x1 + x2 + u
est <- felm(y ~ 1 | 0 | (x1 | x2 ~ z1 + z2))
summary(est)
## Not run:
# summary(est$stage1, lhs='x1')
# summary(est$stage1, lhs='x2')
# ## End(Not run)
# the joint significance of the instruments in both the first stages are ok:
t(sapply(est$stage1$lhs, function(lh) waldtest(est$stage1, ~z1|z2, lhs=lh)))
# everything above looks fine, t-tests for instruments,
# as well as F-tests for excluded instruments in the 1st stages.
# The conditional F-test reveals that the instruments are jointly weak
# (it's close to being only one instrument, z1+z2, for both x1 and x2)
condfstat(est, quantiles=c(0.05, 0.95))
```

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