A function appear only once in the activity hierarchy.
Siblings must be either all functions or all processes.
An elementary process cannot be decomposed.
Each activity that is not an elementary process must decompose into at least two subordinate activities.
The subordinates of an activity must completely describe the activity. For example, if process A decomposes into processes W, X, Y, and Z, then the following statement must be true:
A = W + X + Y + Z
Each leaf of the completed hierarchy must be either a manual process or an elementary process.
Each elementary process must be included in the response to at least one event.
