[Help-glpk] Error : multiplication of linear forms not allowed

[hncp]

Hi,

I'm trying to solve a subset selection problem using GLPK.

I have n connected processes to be deployed in two machines. Each process
has different performance cost for each machine. Initially all the processes
are deployed in machine 1, I need to move subset of these processes to
machine 2 to minimize the total performance cost. If two connected processes
are deployed in different machines there is transfer cost, this also should
be taken into account when calculating the total cost.

Assume processes are connected in the order of P1, P2, ......., Pn . I
represented each process as a binary variable Pi, if the process i to be
moved machine 2, Pi = 1, else 0. 

So my objective function is
minimize: sum {i in Processes} (M1i - (M1i -M2i-Ti)* Pi) where 
M1i - cost of running ith process in machine 1
M2i - cost of running ith process in machine 2
Ti - transfer cost between ith process & (i+1)th process if they are in
different machines

If both Pi & Pi+1 are moved to machine 2 transfer cost between them should
be set to zero. My problem is how to specify this constraint in GLPK ??

I tried to do it as 
minimize: sum {i in Processes} (M1i - (M1i -M2i-(Pi+1)*Ti)*Pi) but it gives
me an error saying "multiplication of linear forms not allowed" 

Is there any other way to do this or is this problem cannot be solved using
GLPK?

Thanks in advance
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