Optional Method dependencies()

This optional method takes no arguments and returns a list of variable names that this variable needs in order to compute its values. Each variable name must be either a fully-qualified Opus variable name, or a dataset-qualified primary attribute name. It is important to list all dependent variables here and don't invoke the computation of the dependent variables from the compute() method, since the dependencies tree mechanism would not work correctly (see Section 7.4.3). If the names of the dependent variables are not known at the time this method is called, the dependencies list can be extended from the compute() method using the Variable method add_dependencies() which takes as an argument a list of additional dependencies (specified either as character strings or in form of AttributeBox). add_dependencies() extends the dependencies list only when the compute() method runs for the first time.