[Help-glpk] How to use less operator in gmpl?

[Andy . Zhu]

Hi, I am new to GNU MathProg language.
I have following model but glpk always complains:

"test2.mod:10: operand preceding
less has invalid type"

The GMPL document says it has this less
operator. How could I use it in modeling? 

Thanks very much for any help.


Andy

# model
set I;
set J;

var a {i in I}, binary;
var b {i in I}, integer, >=0;

maximize Z: sum {i in I diff J} b[i];

s.t. R1 {i in J}: a[i] =0;
s.t. R2 {i in I diff J}: b[i] = a[i]
less a[i-1];
s.t. R4 {i in I diff J}: a[i]-a[i-1]
!=0;

data;
set I : 0, 1, 2, 3, 4, 5;
set J : 0;

end;