    [Note] the argument "a" and "b" tensor defines the order of the out-come bond. After contract,  the order of remaining bonds (both in-bond(bra) and out-bond(ket)) that appears in the new-tensor will follows the rule: a's in-bond will appears first, then the b's in-bond; a's out-bond will appears first, then b's out-bond (see example in below)
