[Ethereum] access struct from mapping in a different contract

I want to access / modify data from a contract from another contract.
If you could help me understand the error that would be great!

I get the following error:

Untitled1:33:8: Error: Indexed expression has to be a type, mapping or array >(is function (address) constant external returns (bool,address,uint256,bool))
        if (a.Owners[msg.sender].sub = false){
            ^------^

below is the code :

pragma solidity ^0.4.7;

contract abc{
    struct Owner{
        bool exist;
        address owner;
        uint share;
        bool sub;
    } // un owner as son address et son percent d'ownership

    mapping (address => Owner) public Owners;
    uint public no_owners; 
    uint public no_sub;


    function abc(){ // a l'execution, l'ownership est transferer a l'excecuteur avec 100 shares.
        //initate
    }

    function transfer(address to,uint amount){
        //transfer ownership
    }
}

contract cde{
    mapping (uint => abc) all_cde;
    uint public f;


    function ask_subdivise(abc a){
        //les different owners vont devoir lancer la fonction
        // une fois que tout les owners on lancer la fonction, le lot est diviser.
        if (a.Owners[msg.sender].sub = false){
            a.Owners[msg.sender].sub=true;
            a.no_sub+=1;

        }
    }
}