How to get block at coordinates?

[12xx12]

You can do it with this code:

local blocktype = world:GetBlock(player:GetPosition():addedY(-1));

I get error attempt to index local 'PlayerWorld' (a nil value)
with this code:

33 function testBlock(aPlayer, OldPosition, NewPosition)
34     local PlayerWorld = OldPosition.world -- I get error here
35     local block = PlayerWorld:GetBlock(aPlayer:GetPosition():addedY(-1))[/font]
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And thank you for your support! 

The Position object doesn't know it's world. it's only three values.
You need to ask the player object for it's world

34     local PlayerWorld = Player:GetWorld()