[boost] Question about Spirit architecture

Hi boost developers, The architecture for spirit is documented under http://www.boost.org/doc/libs/1_38_0/libs/spirit/classic/doc/organization.ht... my question: should all modules of Core (scanner, primitives, composite, non_terminal) also be independent of each other ? so should they be independent subsystems or are they allowed to have dependencies (within the Core layer) ? thanks for your help kind regards, Bernhard.