The maintenance burden

Fixing a missing case took more than writing one new rule.

An expert first described the gap. A knowledge engineer changed the rulebook. The team reran old cases to check that the new rule had not broken an earlier answer. Then the expert tried more cases until another gap appeared.

The developers had hoped rules would remain mostly independent. Instead, experts sometimes needed to inspect related rules and understand how one change would affect the full solution.

Turning a specialist's judgment into a reliable rulebook was called . The slow cycle became one of the central problems in building expert systems.