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 ==(
MathExpr lhs,
MathExpr rhs
)