I changed to accept negative terrain effect (avoidance) values to be set and changed the terrain effect (avoidance) for sand, cliffs and bridges to -5.
When implementing a skill that only activates when the terrain effect (avoidance) value is 0, it was a hindrance to have the terrain effect (avoidance) of sand and cliffs be 0, but it was also unnatural to make the avoidance increase when in those terrains, so I made this change before adding the skill.
If the avoidance value before adding the terrain effect is 4 or less, the value will be 0 (it will not be negative). Also, the unique skill "Covert" has been changed to not double when the terrain effect (avoidance) is negative.
