UnitTypeProperty: vision-bend n
This property is the amount by which a unit can see "around corners".
0 means that vision is strictly line-of-sight,
while 100 means that elevations never obstruct vision.
Defaults to 100.
Table: eye-height u t -> dist
This propety is the additional elevation above the unit's position that a unit
can see with, when in the given terrain.
Defaults to 0.
TerrainTypeProperty: thickness dist
This property is the thickness of the terrain, which is the difference between
the "ground" of the terrain and its top.
Defaults to 0.