(a) Unless a section in this article otherwise provides, the effect of any provision of this article may be varied by agreement.

(b) The presence of mandatory language, such as “must” or “shall”, or the absence of enabling language, such as “unless otherwise agreed”, does not by itself preclude the parties from varying by agreement a provision of this article.

(c) Whenever this article allocates a risk or imposes a burden as between the parties, they may agree to shift the allocation and apportion the risk or burden.