Status: IN
Visitor's dual class hierarchy design (one for elements, one for visitors) is the structural mechanism enabling computation across elements that share no common parent class: because the visitor hierarchy is orthogonal to the element hierarchy, visitors can traverse and accumulate state across structurally unrelated objects that merely implement Accept.
Dual hierarchy architecture directly enables the no-common-parent capability
Depends on (SL): gof-visitor-two-class-hierarchies, gof-visitor-no-common-parent-required