diff --git a/src/ecs.lua b/src/ecs.lua
index bb21600..10f6fbf 100644
--- a/src/ecs.lua
+++ b/src/ecs.lua
@@ -2,19 +2,19 @@ ecs = { }
 
 local ALL_ENTITIES = {}
 
----@alias System { callback: fun(delta: number, a: any, b: any, c: any, d: any, e: any, any), shapes: {}, keys: string[], entityCache: table<any, boolean> }
+---@alias System { callback: fun(delta: number, entity: any, a: any, b: any, c: any, d: any, e: any, any), shapes: {}, keys: string[], entityCache: nil | table<any, boolean> }
 
 ---@type System[]
 local SYSTEMS = {}
 
-local PLACEHOLDER = {}
-
 -- TODO: Add entity to any existing systems
 function ecs.addEntity(entity)
 	ALL_ENTITIES[entity] = true
 	for _, system in pairs(SYSTEMS) do
 		if entityMatchesShapes(entity, system.shapes) then
 		    system.entityCache[entity] = true
+		else
+		    system.entityCache[entity] = nil
 		end
 	end
 end
@@ -26,7 +26,9 @@ function ecs.removeEntity(entity)
 	end
 end
 
----@return any
+local PLACEHOLDER = {}
+---@generic T
+---@return T
 function ecs.field()
 	return PLACEHOLDER
 end
@@ -89,6 +91,11 @@ function addSystem(callback, keys, shapes)
 	}
 end
 
+---@return boolean
+function is(entity, shape)
+	return allKeysIncluded(entity, shape)
+end
+
 ---@param deltaSeconds number
 function ecs.update(deltaSeconds)
 	for _,system in pairs(SYSTEMS) do
@@ -100,9 +107,9 @@ function ecs.update(deltaSeconds)
 				end
 			end
 		end
+		local keys = system.keys
 		for entity,_ in pairs(system.entityCache) do
-			local keys = system.keys
-			system.callback(deltaSeconds, entity[keys[1]], entity[keys[2]], entity[keys[3]], entity[keys[4]], entity)
+			system.callback(deltaSeconds, entity, entity[keys[1]], entity[keys[2]], entity[keys[3]], entity[keys[4]], entity)
 		end
 	end
 end
@@ -118,11 +125,11 @@ end
 ---@generic W
 ---@generic WKey
 ---@param tShape { [TKey]: T }
----@param uShape { [UKey]: U } | fun(componentT: T, any) | nil
----@param vShape { [VKey]: V } | fun(componentT: T, componentU: U, any) | nil
----@param wShape { [WKey]: W } | fun(componentT: T, componentU: U, componentV: V, any) | nil
----@param finalFunc fun(componentT: T, componentU: U, componentV: V, componentW: W, any) | nil
-function ecs.entitiesWithShapes(tShape, uShape, vShape, wShape, finalFunc)
+---@param uShape { [UKey]: U } | fun(entity: any, componentT: T, any) | nil
+---@param vShape { [VKey]: V } | fun(entity: any, componentT: T, componentU: U, any) | nil
+---@param wShape { [WKey]: W } | fun(entity: any, componentT: T, componentU: U, componentV: V, any) | nil
+---@param finalFunc fun(entity: any, componentT: T, componentU: U, componentV: V, componentW: W, any) | nil
+function ecs.forEntitiesWith(tShape, uShape, vShape, wShape, finalFunc)
 	local maybeShapes = {tShape, uShape, vShape, wShape, finalFunc}
 	local shapes = {}
 	local callback
@@ -146,23 +153,59 @@ function ecs.entitiesWithShapes(tShape, uShape, vShape, wShape, finalFunc)
 end
 
 local f = ecs.field()
-
----@type { x: number, y: number }
 local XYPair = { x = f, y = f }
-
 local Position = { position = XYPair }
+local Target = { target = XYPair }
 local Velocity = { velocity = XYPair }
 
-local someMovable = {
-	position = { x = 0, y = 0 },
-	velocity = { x = 2, y = 1 }
+function ecs.overlayOnto(entity, value)
+	for key,v in pairs(value) do
+		entity[key] = v
+	end
+	ecs.addEntity(entity)
+end
+
+local data = {
+	position = {x = 0, y = 0},
+	velocity = {x = 1, y = 2},
 }
 
-ecs.addEntity(someMovable)
+---@generic T
+---@param shape T
+---@param entity unknown
+---@return T
+function ecs.get(shape, entity)
+	return entity
+end
 
-ecs.entitiesWithShapes(Position, Velocity, function (delta, pos, vel)
-	print("position:")
+---@generic T
+---@param entity unknown
+---@param shape `T`
+---@param value `T`
+function ecs.set(entity, shape, value)
+	for key,v in pairs(shape) do
+		entity[key] = value[v]
+	end
+end
+
+ecs.addEntity(data)
+
+ecs.forEntitiesWith(Position, Velocity, function (delta, e, pos, vel)
+	pos.x = pos.x + (delta * vel.x)
+	pos.y = pos.y + (delta * vel.y)
+	print("position")
 	tprint(pos, 1)
-	pos.x = pos.x + (vel.x * delta)
-	pos.y = pos.y + (vel.y * delta)
-end)
\ No newline at end of file
+	ecs.set(Target, e, {
+		--target = { x = 10, y = 10}
+	})
+end)
+
+ecs.forEntitiesWith(Target, function (delta, e, pos, vel)
+	pos.x = pos.x + (delta * vel.x)
+	pos.y = pos.y + (delta * vel.y)
+	print("position")
+	tprint(pos, 1)
+	ecs.set(e, Target, 'hallo')
+end)
+
+ecs.update(1)
\ No newline at end of file