disputed by Pollack et al. (1997). They instead attribute much of the power of ZLIFO to its delaying of separable threats, and propose a variation of LCFR, LCFR-DSep, that also delays separable threats. Since we chose to work with ground **actions at IPC3, separability**
was not an issue for us.

## 3.2.1 Notation For Specifying Flaw Selection Strategies

In order to better understand the differences between various flaw selection strategies, and to simplify comparative studies, Pollack et al. (1997) proposed a unifying notation for specifying flaw selection strategies. We adopt their notation with only slight modifications.

A flaw selection strategy is an ordered list of selection criteria. Each selection criterion is of the form
{flaw types}≤**max refinementsordering criterion**,
and applies to flaws of the given types that can be resolved in at most **max refinements**
ways. If there is no limit on the number of refinements, we simply write

## {Flaw Types}**Ordering Criterion**.

The ordering criterion is used to order flaws that the selection criterion applies to. LIFO
order is used if the ordering criterion cannot be used to distinguish two or more flaws.

Pollack et al. define the flaw types "o" (open condition), "n" (non-separable threat),
and "s" (separable threat). They also define the ordering criteria "LIFO", "FIFO", "R"
(random), "LR"3**(least refinements first), and "New". The last one applies only to open**
conditions, and gives preference to open conditions that can be resolved by adding a new action. The rest apply to both open conditions and threats.

Flaws are matched with selection criteria, and it is required for completeness that every flaw matches at least one selection criterion in a flaw selection strategy. The flaw that matches the earliest selection criterion, and is ordered before any other flaws matching the same criterion (according to the ordering criterion), is the flaw that gets selected by the flaw selection strategy. Note that we do not always need to test all flaws. If, for example, the first selection criterion is {n,s}**LIFO, and we have found a threat, then we do not need**
to consider any other flaws for selection.

Using this notation, we can specify many different flaw selection strategies in a concise manner. Table 3 specifies the flaw selection strategies mentioned earlier. A summary of flaw types recognized by VHPOP**, including three new flaw types defined below, is given**
in Table 4.

## 3.2.2 New Flaw Selection Strategies

We now propose several additional flaw types and ordering criteria, and use these in combination with the previous ones to obtain some novel flaw selection strategies. Four of these new flaw selection strategies were used at IPC3 and contributed to the success of VHPOP
at that event.