6. A contractual term shall be considered to be unilaterally imposed within the meaning of this Article if it has been supplied by one contracting party and the other contracting party has not been able to influence its content despite an attempt to negotiate it. The contracting party that supplied the contractual term bears the burden of proving that that term has not been unilaterally imposed. The contracting party that supplied the contested contractual term may not argue that the term is an unfair contractual term.