Note that a constraint is composed of a constraint expression and forall sets expression.
ConstraintExpr expr = sum(over(j), x[i, j]) <= sum(over(j), x[j, i]);
Constraint con = forall(i) | sum(over(j), x[i, j]) <= sum(over(j), x[j, i]);
public static ConstraintExpr operator <=(
Summation lhs,
Summation rhs
)