diff --git a/custom_functions_you_can_override.lua b/custom_functions_you_can_override.lua
index f935455..9bb0fb2 100644
--- a/custom_functions_you_can_override.lua
+++ b/custom_functions_you_can_override.lua
@@ -204,7 +204,6 @@ yl_speak_up.custom_functions_r_ = {}
 
 -- example function for preconditions:
 yl_speak_up.custom_functions_p_[ "example function" ] = {
-	function_call = "yl_speak_up.custom_example_function",
 	description = "This function is just an example. It tells the player its parameters.",
 	param1_text = "1. Parameter:",
 	param1_desc = "This is the value passed to the function as first parameter.",
@@ -224,17 +223,36 @@ yl_speak_up.custom_functions_p_[ "example function" ] = {
 	param8_desc = "This is the value passed to the function as 8. parameter.",
 	param9_text = "9. Parameter:",
 	param9_desc = "This is the value passed to the function as 9. parameter.",
+	-- the actual implementation of the function
+	code = function(player, n_id, p)
+		local pname = player:get_player_name()
+		local str = ""
+		for i = 1,9 do
+			str = str.."\n\tParameter "..tostring(i)..": "..tostring(p["p_param"..tostring(i)])
+		end
+		minetest.chat_send_player(pname, "Checking precondition "..tostring(p.p_id)..
+			" for NPC with ID "..tostring(n_id)..": Executing custom function "..
+			tostring(p.p_value).." with the following parameters:"..
+			str.."\n(This function just tells you its parameters and returns true.)")
+		-- this function is just for demonstration; it always returns true
+		return true
+	end,
 }
 
-yl_speak_up.custom_functions_p_[ "get_level_of_player" ] = {
-	function_call = "yl_speak_up.get_player_level",
-	description = "Get the level the player has achieved.",
+yl_speak_up.custom_functions_p_[ "get_xp_of_player" ] = {
+	description = "Get the xp the player has achieved (requires xp_redo).",
+	code = function(player, n_id, p)
+		local pname = player:get_player_name()
+		if(minetest.get_modpath("xp_redo")) then
+			return xp_redo.get_xp(pname)
+		end
+		return 0
+	end
 }
 
 
 -- TODO: actually implement these as examples
 yl_speak_up.custom_functions_r_[ "set_variable_to_random_number" ] = {
-	function_call = "yl_speak_up.set_variable_to_random_number",
 	description = "Set a variable to a random number.",
 	param1_text = "Name of the variable:",
 	param1_desc = "Which variable do you want to set to a random number?",
@@ -245,7 +263,6 @@ yl_speak_up.custom_functions_r_[ "set_variable_to_random_number" ] = {
 }
 
 yl_speak_up.custom_functions_r_[ "set_variable_to_random_value_from_list" ] = {
-	function_call = "yl_speak_up.select_random_value_from_list",
 	description = "Set a variable to a random value from a list.",
 	param1_text = "Name of the variable:",
 	param1_desc = "Which variable do you want to set to a random value from your list?",
@@ -257,7 +274,6 @@ yl_speak_up.custom_functions_r_[ "set_variable_to_random_value_from_list" ] = {
 
 -- example function for results/effects:
 yl_speak_up.custom_functions_r_[ "example function" ] = {
-	function_call = "yl_speak_up.custom_example_function",
 	-- Describe here in short form what your function does:
 	description = "This function is just an example. It tells the player its parameters.",
 	param1_text = "1. Parameter:",
diff --git a/exec_eval_preconditions.lua b/exec_eval_preconditions.lua
index 9099a46..091e203 100644
--- a/exec_eval_preconditions.lua
+++ b/exec_eval_preconditions.lua
@@ -262,7 +262,18 @@ yl_speak_up.eval_precondition = function(player, n_id, p, other_options_true_or_
 		return yl_speak_up.eval_precondition_with_operator(p, properties[p.p_value])
 
 	elseif(p.p_type == "evaluate") then
-		return true -- TODO
+		if(not(player) or not(p.p_value)) then
+			return false
+		end
+		local custom_data = yl_speak_up.custom_functions_p_[p.p_value]
+		if(not(custom_data) or not(custom_data.code)) then
+			return false
+		end
+		local fun = custom_data.code
+		-- actually call the function
+		local ret = fun(player, n_id, p)
+		-- compare the result with wat is expected
+		return yl_speak_up.eval_precondition_with_operator(p, ret)
 	elseif(p.p_type == "block") then
 		if(not(p.p_pos) or type(p.p_pos) ~= "table"
 		  or not(p.p_pos.x) or not(p.p_pos.y) or not(p.p_pos.z)) then
diff --git a/fs_edit_preconditions.lua b/fs_edit_preconditions.lua
index 1566be7..c77500d 100644
--- a/fs_edit_preconditions.lua
+++ b/fs_edit_preconditions.lua
@@ -246,8 +246,9 @@ yl_speak_up.show_precondition = function(p, pname)
 				str = str..","
 			end
 		end
+		local i_op = math.max(1,table.indexof(values_operator, p.p_operator))
 		return "FUNCTION["..tostring(p.p_value).."]"..
-			"("..str..") "..tostring(check_operator[p.p_operator])..
+			"("..str..") "..tostring(check_operator[i_op])..
 			" "..tostring(p.p_var_cmp_value)
 	elseif(p.p_type == "block") then
 		if(not(p.p_pos) or type(p.p_pos) ~= "table"