[Help-glpk] Transport Problem advanced

[SpaceRider]

// s.t. supply{i in I}: sum{j in J} x[i,j] = a[i];
s.t. supply{i in I}: sum{j in J} x[i,j] = a[i]; // process all offer

s.t. demand{j in J}: sum{i in I} x[i,j] >= b[j];

Now, I need a additional comparison, but not a range from to
sum{i in I} x[i,j] >= b[j] OR sum{i in I} x[i,j] = 0

How I have to write the syntax?
Is it still a LP-Problem?

Can someone help me?

Thank you
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