ZoC Purity (Server setting)

Note: zoc_purity is a setting on Freeciv-Web server. It is not yet included in any other Freeciv server.
Mechanics:
If enabled, igZOC units do not negate ZoC for other units. (An igZOC unit is a unit which ignores ZoC. Aircraft, diplomats, explorers, etc.)
Article: 
===Why zoc_purity is an evolutionary advance to game mechanics.===
To understand why zoc_purity is evolutionary, we must look at the historical development of Civ-like games:
1. Stage One. The great ancestors wanted a war game with tactics. Tactical positioning was important. Armies occupy hills, rivers, and other tiles which block nearby movement and control where battles can take place. If you already occupy a tile, you can move to and from occupied tiles. This sets the stage for important battle lines and tactical terrain control. This is all excellent! It allows positioning of units like a true war game, allowing players to be generals with tactical thinking. But...
2. Stage Two. Before release of even the first Civ game, it was obvious that Stage One mechanics are too simplistic. The inability for anyone to move past ZoC is unrealistic and too restrictive. Shouldn't some units be immune to ZoC? Like sneaky spies, explorers, or partisans who know the ravines and shortcuts? And of course, airplanes in the air who fly over? Certainly! That makes sense. But ...
3. Stage Three. The meaningfulness of ZoC and tactical positioning was so nerfed by Stage Two, that in the late game it was all but eliminated.  All it takes is a lone spy or aircraft to allow the entire 149th Panzer Division to slip through massively fortified battle lines and speed down the highway. Any late game nation of any economic means can muster a lone spy, fighter, or partisan. Put this unit on any tile, and now the entire army completely ignores the existence of any ZoC and battle lines. In almost any case, a unit who ignores ZoC can be available to make ZoC completely meaningless. In other words, mid- and late-game wars effectively lose all the chess-like importance of true battle tactics. Freeciv got stuck in Stage Three, tinkering with a poor solution: 
3a. A certain school of thought held ZoC and igZoC rules as holy and sacrosanct. They had to think of something else. Which they did... a hack called RestrictINFRA which magically makes enemy units unable to use roads. Now, even though it was unrealistically easy to slip through front lines, it was also unrealistically hard for units to use any kind of road infra at all. One ridiculous imbalance was corrected by another. In a way, it was now balanced again. But in another way, it was now two unrealistic horrible imbalances canceling each other out. Even so, it was no longer possible for an AWACS to fly to a special tile that lets an entire Division slip past heavily fortified battle fronts and use the railroad to take over the entire interior of a nation. But...
3b. Another school of thought resisted and rejected correcting one unrealistic imbalance with another unrealistic imbalance. They took a subtle nod in the right direction and 1) reduced rail speed, and 2) acknowledged that using enemy rail lines is simple for any defender to disallow. This made RestrictINFRA actually work in a playable more balanced way, BUT, it only fixed the rail problem. It left the problem of "magic Panzers" slipping onto interior highways by using the magic carpet ride.
4. Stage Four. It is insular to suppose that Freeciv is the only game that has ever thought of these problems. Other solutions have been incorporated in later releases of the commercial series, and indeed even in free software as old as C-evo. With the exception of Civ V, the most common solution they have all used is the concept of ZoC Purity. The server setting zoc_purity finally brings this to Freeciv. Of course, this solution is introduced as a server setting so that it can be optional. But it is really intended to advance Freeciv beyond the antiquated ZoC rules like other Civ-like games have done many years ago. It really ought to be the new standard in future mainstream rulesets.
zoc_purity is a simple idea that fixes the original mistake that took place in Stage Two. Units with special "igZoC-slipperiness" should not magically confer that to units who aren't "slippery". For example, an AWACS in the air should not fly to the Maginot line to allow the entire 28th Division to slip diagonally through the frontline without a single shot fired. The AWACS remains slippery up at high altitude and can go there, just like a Partisan hiding in ravines and trees, or a spy in disguise. But none of these slippery units should allow entire armies to magically pass through fortified battle lines like they don't exist. When zoc_purity is enabled, "slippery" units can still go wherever they please, but they don't negate ZoC for conventional units. Conventional units can still go to any tile occupied by other conventional units. This finally allows all the mechanics from Stage One to live and breathe again.
zoc_purity simply means that igZOC units lose their special "magic carpet" for slipping entire armies past battle lines. Interestingly, this doesn't nerf their importance. In some ways they are more important than before, as they are now the only units who can get past ZoC lines. Aircraft, spies, and partisans have the exclusive ability to get past lines to do attacks, sabotage, or other tactical missions.
zoc_purity finally allows everything good from Stage One, Stage Two, and Stage Three, to all co-exist together, without one of the mechanics nullifying or overcorrecting the other. It finally brings Freeciv capabilities to levels other Civ-like games had for many years.
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