With combination subsets, the Analyst can combine multiple subsets using the set operators add, subtract, union and intersection. An initial state can be defined for the subset (so that the subsets can be populated by subtracting other subsets from the entire design). The Analyst can also define the hierarchical rollup of lower-level subsets (this is the only type of subset that references lower-level subsets).
How To Create a Combination Subset
1
Right-click on the top-level "SUBSETS" folder in the Explorer Tree and choose "New Subset..." from the pop-up context menu.
Alternative: Choose "New.." from the Subsets submenu within the Design menu.
2
On the Create New Subset dialog, type in the name to be given to the new subset, select the subset type labeled "Combination" and then click on the "OK" button.
How To Define an Combination Subset
1
In the Standard Editing Mode, click on the selection subset.
2
Choose the Properties tab within the context panels.
3
On the Properties panel, select the desired initial state for the subset. This will be a pre-fill instruction that will be evaluated prior to filling the combination subset with elements from other subsets.
4
Add instructions by clicking on the Operator dropbox and choosing a set operator (described below), then clicking on the Referenced Subset drop box and choosing an existing subset.
For each instruction, the Analyst can choose from the following four set operators:
Operator
Pass
Interpretation
Union
1
add the contents of the instruction's subset to the current contents of the combination subset
Intersection
1
remove from the combination subset any elements that are not included in the instruction's subset
Add
2
add the contents of the instruction's subset to the current contents of the combination subset
Subtract
2
subtract the contents of the instruction's subset from the current contents of the combination subset
When eXpress populates a combination subset, it first fills out the initial state of the subset (based on the setting selected on the Properties panel), then evaluates the instructions in the order they are listed, adding or subracting elements to the combination subset based on the set operator defined for each instruction. Instructions are evaluated in two passes. In the first pass, Union and Intersection instructions are applied sequentially to the combination subset; in the second pass, all Add and Subtract instructions are evaluated (the only difference between a Union and Add operator is that they are performed in different passes).
