How can I pass function to formulas?

Question

guys: I want to use systemfit to estimate a set of formulas which is composed by two parts,as the picture showing:

there are four equations needed to be estimated,so I have codes :

 library(systemfit)
library(micEconAids)
data( Blanciforti86 )

eps <- list()
eps1 <- formula(wFood1 ~ Xfoodd + pFood1)
eps2 <- formula(wFood2 ~ Xfoodd + pFood2)
eps3 <- formula(wFood3 ~ Xfoodd + pFood3)
eps2 <- formula(wFood4 ~ Xfoodd + pFood4)

#how can I write xFoodd ?

fit <- systemfit(eps,"SUR")

My question is how to write the code of xFoodd,it seems that I can write a function of lnp and xfoodd,but I don't know how to do that.I wonder if you can give me some guides. Many Thanks!