Previously, ProblemBlock's implementation was split between CapaMixin/CapaFields (in capa_base.py) and ProblemBlock (in capa_module.py), the former being the base classes of the latter. The split existed for a historical reason: as a former XModule, ProblemBlock was once split between CapaDescriptor (author-facing) and CapaModule (learner-facing). Since ProblemBlock has been converted to being a pure XBlock, the division between the base classes and the block class are no longer necessary nor semantically helpful. docs: Flesh out ProblemBlock's docstring a bit.
common ------ This directory contains common code shared between LMS and CMS, such as Mako templates, CSS, and Coffescript.