mixture variables

Dear list members,

I want to fit a linear model to a data set that contains several independent
variables, some of them continuous and some of them binary. For the binary
variables I am using 1-of-N encoding but I also have two variables that
correspond to mixture ratios. How can I declare to the model that their sum
equals unity? Or perhaps should I treat them as factors instead of  random
variables constrained to [0,1]?
Thank you very much in advance.

Best Regards
Kyriakos Kachrimanis.

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