How can I apply constraints to coefficients in SUR regression with systemfit package in R

Question

I am trying to fit a SUR model with systemfit package, and I want to make sure some of the coefficients in equation 1 and 2 to be equal.

example:

f1 <- y ~ x1 + x2
f2 <- y ~ x3 + x4
system <- list(eq1 = f1 , eq2 = f2)
model <- systemfit(system, "SUR", data = data, maxit = 100)

How can I constrain the coefficients of x1 and x4 to be equal?