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Polymorphism.sol

pragma solidity ^0.4.0;

interface Letter {
    function n() external returns (uint);
}

contract A is Letter {
    function n() public returns (uint) {
        return 1;
    }
}

contract B is Letter {
    function n() public returns (uint) {
        return 2;
    }
}

contract C is Letter {
    function n() public returns (uint) {
        return 3;
    }
}

contract Alphabet {
    Letter[] private letters;
    
    event Printer(uint);
    
    constructor() public {
        A a = new A();
        
        letters.push(a);
        letters.push(new B());
        letters.push(new C());
    }
    
    function print() public {
        for (uint i = 0; i < letters.length; i++) {
            emit Printer(letters[i].n());
        }
    }
}