  Developing complex software requires that multiple views and versions of the
software can be developed in parallel and merged as supported by views and
managed by version control systems.
  In this context, this paper considers monitoring merging and related
consistency problems permanently at the level of models and abstract syntax to
permit early and frequent conflict detection while developing in parallel. The
presented approach introduces multi-version models based on typed graphs that
permit to store changes and multiple versions in one graph in a compact form
and allow (1) to study well-formedness for all versions without the need to
extract each version individually, (2) to report all possible merge conflicts
without the need to merge all pairs of versions, and (3) to report all
violations of well-formedness conditions that will result for merges of any two
versions independent of any merge decisions without the need to merge all pairs
of versions.
  The paper defines the related concepts and algorithms operating on
multi-version models, proves their correctness w.r.t.~the usually employed
three-way-merge, and reports on preliminary experiments concerning the
scalability.
