Automatically merged updates to draft EIP(s) 1155 (ethereum#2120)

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 - It only modifies existing Draft or Last Call EIP(s)
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EIPS/eip-1155.md

@@ -424,6 +424,7 @@ If the optional `ERC1155Metadata_URI` extension is included:
     - An implementation MAY emit the `URI` event during a mint operation but it is NOT mandatory. An observer MAY fetch the metadata uri at mint time from the `uri` function if it was not emitted.    
 * The `uri` function SHOULD be used to retrieve values if no event was emitted. 
 * The `uri` function MUST return the same value as the latest event for an `_id` if it was emitted.
+* The `uri` function MUST NOT be used to check for the existence of a token as it is possible for an implementation to return a valid string even if the token does not exist.
 
 ```solidity
 pragma solidity ^0.5.9;