Summary: ``Modal Meta Model Checking $(M3C)$ is a method and tool supporting meta-level product lining and evolution that comprises both context-free system structure and modal refinement. The underlying Context-Free Modal Transition Systems (CFMTSs) can be regarded as loose specifications of meta models, and modal refinement as a way to increase the specificity of allowed domain specific languages (DSLs) by constraining the range of allowed syntax specifications. Model checking with $M3C$ allows one to verify properties specified in a branching-time logic for all DSLs of a given level of specificity in one go. The paper illustrates the impact of $M3C$ in an industrial setting where well-formed documents serve as contracts between a provider and its customers in two steps: it establishes CFMTS as a formalism to specify product lines of document description types (DTDs---or related formalisms like JSON schema), and it shows how $M3C$-based product line verification can be used to guarantee that violations of essential well-formedness constraints of a corresponding user document are detected by standard DTD-checkers. The resulting hierarchical product line verification allows Creios GmbH, a service provider for E-commerce systems to provide a wide range of tailored shop applications whose essential business rules are checked by a standard DTD-checker.''