I have the following inequality system : 

              [ U_1,2 + U_1,3 -1 = 0,

U_2,3 - U_1,2 = 0, 

-U_1,3 -U_2,3+1 = 0,

0 <=U_1,2,

0 <= U_1,3,

0 <= U_2,3]

I want to solve it using LinearMultivariateSystem, namely I excuted a command : 

Then the following is return : 

Error, (in Utilities:-SimpleAnd) invalid input: a string/name list is expected for sort method `lexorder`

 I tried that

My question is that how do you use LinearMultivariateSystem for varibles indexed by two params.

Thank you.