Allowed for entities to be in multiple systems. Fixed the removal issue.

fluid/entity.lua

@@ -1,3 +1,7 @@
+local PATH = (...):gsub('%.[^%.]+$', '')
+
+local List = require(PATH..".list")
+
 local Entity = {}
 Entity.__index = Entity
 
@@ -6,7 +10,8 @@ Entity.__index = Entity
 function Entity.new()
    local e = setmetatable({
       components = {},
-      instance   = nil,
+      removed    = {},
+      instances  = List(),
    }, Entity)
 
    return e
@@ -26,15 +31,27 @@ end
 -- @param component The Component to remove
 -- @return self
 function Entity:remove(component)
-   self.components[component] = nil
+   self.removed[component] = true
 
    return self
 end
 
+function Entity:destroy()
+   for i = 1, self.instances.size do
+      self.instances:get(i):removeEntity(self)
+   end
+end
+
 --- Checks the Entity against the pools again.
 -- @return self
-function Entity:check()
-   self.instance:checkEntity(self)
+function Entity:apply()
+   for i = 1, self.instances.size do
+      self.instances:get(i):checkEntity(self)
+   end
+
+   for _, component in pairs(self.removed) do
+      self.components[component] = nil
+   end
 
    return self
 end