Tool activation assumes that a well-formed causal model with determinant dependencies exists. At any time a sketch might or might not be well formed and complete. Each time you activate a tool, the current sketch is checked to determine its conformance to these requirements.
If the sketch is lacking in any area (for example, missing an equation), Vensim applies interim patches. You will not receive a message that something is wrong, but you might notice that, for example, the Document tool shows an A FUNCTION OF equation indicating incomplete formulation. The menu Model>Check Model command displays any hidden error messages.
