[Help-glpk] How to get a variable's absolute value with GNU mathprog

[hhb83]

Hello, 
Would anyone tell me how to get a variable's absolute value with GNU
mathprog? 
var X;
var Xabs;
s.t. absolte: Xabs = abs(X);
It always says: argument for abs has invalid type.

If I write this:
s.t absolte: Xabs = if X<=0 then (-1)*X else X;
It says: operand proceding <= has invalid type.
Then what should I do?
Thank you!
Haibin




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