The presence of a bridge node within a PPB complicates the assignment of direction (Figure ). A bridge node has no children and therefore can never be a parent. At the same time, a bond always points from parent to child. This means that a bridged PPB yields a pair of bonds with opposite directions. The This problem can be addressed by hydrogen reification followed by partial parity assignment to the newly-available carbon-hydrogen bonds (right). first (or "left") bond, has the direction that would result from a direct bond to an atomic child. The second (or "right") bond has the opposite direction. Cyclic conjugated polyenes present special challenges to the use of PPBs, as exemplified by cyclooctatetraene (Figure ). Although the (syn, syn, syn, syn) conformation is expressable using PPBs located along carbon-carbon axes only, the (syn, syn, syn, anti ) conformation is not. This problem arises because the conformation for two different double bonds is set by the same PPB.