Nested model structure. Illustration of the hierarchical modeling concept. Different shapes of nodes correspond to different species names while attributes are color-coded. Stacking of identical nodes depicts the amount of a certain species. (A) Graphical representation of a hierarchical model structure via nested nodes. (B) The same model structure alternatively depicted as a directed tree graph. Please note that besides atomic species (triangles and diamonds) also species containing a sub-solution (squares) might be attributed so that each species at each level might has its own state. (C) Examples of matching different reactant patterns within the hierarchical model structure. The rainbow shadings in the second and third pattern illustrate variable instead of defined colors, i.e. attributes.