I have a question related to ERC1820 : Pseudo-introspection Registry Contract
in the offical link (http://eips.ethereum.org/EIPS/eip-1820)
in ERC1820Registry implementation contract what is the reason for defining mapping(address => mapping(bytes32 => address)) internal interfaces;
Why can't it defined as mapping(bytes32 => address) which provides the address of contract which implements the given _intefaceHash argument.
I m missing anything imp here?