This method returns an elementary cumul function expression that is constant over a certain interval and null before that.

This method returns an elementary cumul function expression that, whenever interval variable a is present, is equal to a 0 before the start of a and equal to a value v such that vmin <= v <= vmax after the start of a. The choice of the value v in the range [vmin,vmax] is a decision of the problem. When interval variable a is absent, the function is the constant null function.