This includes normal form theorems for a free product with amalgamation and for an HNN extension (Bass 1993).
These are modeled on orbifolds arising from cocompact properly discontinuous actions of discrete groups on 2-dimensional simplicial complexes that have the structure of CAT(0) spaces.
The quotient of the simplicial complex has finite stabilizer groups attached to vertices, edges and triangles together with monomorphisms for every inclusion of simplices.
Developability is a non-positive curvature condition on the complex of groups: it can be verified locally by checking that all circuits occurring in the links of vertices have length at least six.
Such complexes of groups originally arose in the theory of 2-dimensional Bruhat–Tits buildings; their general definition and continued study have been inspired by the ideas of Gromov.