key_mergetypes problem on item cmdsets

[GoogleCodeExporter]

I'm still having problems with the same sort of thing as Issue #391

Here's the steps to repeat it:
1) Put this in game/gamesrc/testcmdset.py
----
from ev import CmdSet

class TestCmdSet(CmdSet):
    key = "TestCmdSet"
    priority = 29
    mergetype = "Replace"
    key_mergetypes = {
        'Player' : 'Union',
    }
    no_exits = True
    no_objs = True
    no_channels = False

    def at_cmdset_creation(self):
        """
        Populates the cmdset
        """
        pass
----
2) Create TESTOBJ, set call:true, and put it next to your character
3) @py 
ev.search_object('#5654')[0].cmdset.add('game.gamesrc.testcmdset.TestCmdSet')
4) Now I can't use any commands, either from the Player cmdset or the Character 
cmdset. I expected Player commands to still be available.
5) Change the contents of game/gamesrc/testcmdset.py and @reload
----
from ev import CmdSet

class TestCmdSet(CmdSet):
    key = "TestCmdSet"
    priority = 29
    mergetype = "Replace"
    key_mergetypes = {
        'Character' : 'Union',
    }
    no_exits = True
    no_objs = True
    no_channels = False

    def at_cmdset_creation(self):
        """
        Populates the cmdset
        """
        pass
----
6) Now every command is back to being available. I can still use commands from 
my Character cmdset, and from my Player cmdset.

Original issue reported on code.google.com by daniel@benoy.name on 19 Dec 2013 at 5:32

[GoogleCodeExporter]

You are attempting an impossible combination by trying to merge a cmdset with a 
high priority onto a stack yet expecting it to behave differently with a cmdset 
further down in that stack.

Here is the merge order (ignoring channel/exit cmdsets): 
PlayerCmdSet (prio -5, Union)
CharacterCmdSet (prio 0, Union)
TestCmdSet (prio 29, Replace, Union for PlayerCmdSet)

What happens here is that these are merged in reversed priority order: 
CharacterCmdSet is merged onto the PlayerCmdSet to form a new, merged set named 
CharacterCmdSet. Onto this is then merged TestCmdSet, but since it is merging 
onto the merged set (named CharacterCmdSet) it will not trigger its special 
condition, which was set for PlayerCmdSet. 

TestCmdSet and its key_mergetype exception will only work on the cmdset it is 
actually merging with, i.e. CharacterCmdSet. If you want it to operate on 
PlayerCmdSet further town in the stack), you need to insert it in the right 
place of the priority queue (give it say, priority=-4). 

So what you want to do is not possible with this combination of permissions and 
stack. custom mergetypes were originally intended to be used mainly on simpler 
merges, on top of object-level cmdsets. I can see why it would be confusing, 
but I'm not sure how one would do the merging to have it work the way you want 
it to. 

I'll try to expand the docs on just how mergetype comes into the merge order. 

(The code also had a bug which made it impossible to insert custom cmdsets with 
priorities<0, this is fixed in latest push. This was due to the Empty sets 
having prio 0 and getting weeded out. Empty sets have prio -100 now instead)

Original comment by griatch on 4 Jan 2014 at 5:13

• Changed state: Reviewing