The Vensim modeling language allows you to define the model you want by writing a set of mathematical equations and expressions. You can enter equations (or expressions) in any order. When the model is simulated, the ordering of equations necessary for computation is determined as described above. If this is not possible because of simultaneous initial or active equations, this will be reported. See Model and Unit Checking, Sketch Editor, Equation Editor, and Text Editor for further descriptions of problems and the correction process.