[R] restricted model estimation

[n.vialma at libero.it]

Dear all,
I would like to know how to estimate in R a restricted model. 
The model would be:

y=beta*X1+(1-beta)*X2

so the sum of coefficients must be one. I wonder if there is an option in the 
lm function that allows to specify that restriction or any other solutions to 
get the expected results.

Thanks for your attention!