When a model is implemented in a conventional programming language, large chunks of the program can be enclosed inside an if...endif block: i.e. whether it is actually evaluated depends on some condition. This programming device may be applied to several different purposes:
All these situations can be handled in Simile using a conditional submodel. This is simply a normal submodel, but with a condition symbol added. Visually, we can tell that it's a conditional submodel both by the presence of the condition symbol, and by a set of dots going down diagonally to the right from the submodel envelope. The condition contains a boolean expression: if this evaluates to true, then the submodel (or an instance of it) exists; if not, then it doesn't.
A conditional submodel will, like any other, have influences coming out from the model elements it contains. However, the number of values passed along each influence will either be zero (if the submodel does not exist), or one, if it does. This is thus a variable-size data structure: in other words, a list (with the name of the variable enclosed in curly braces {...} ). In Simile, the only thing that can be done with a list is to evaluate it: usually, to sum its values. If the list is empty, then the sum is zero. If the list contains a single element, then the sum is whatever this value is.