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ERC20.sol

pragma solidity ^0.4.0;

interface ERC20 {
    function totalSupply() constant returns (uint _totalSupply);
    function balanceOf(address _owner) constant returns (uint balance);
    function transfer(address _to, uint _value) returns (bool success);
    function transferFrom(address _from, address _to, uint _value) returns (bool success);
    function approve(address _spender, uint _value) returns (bool success);
    function allowance(address _owner, address _spender) constant returns (uint remaining);
}

MyToken.sol

pragma solidity ^0.4.0;

import "browser/ERC20.sol";

contract MyFirstToken is ERC20 {
    string public constant symbol = "MFT";
    string public constant name = "My First Token";
    uint8 public constant decimals = 18;
    
    uint private constant __totalSupply = 1000;
    mapping (address => uint) private __balanceOf;
    mapping (address => mapping(address => uint)) private __allowances;
    
    constructor() public
    {
        __balanceOf[msg.sender] = __totalSupply;
    }
    
    function totalSupply() constant returns (uint _totalSupply)
    {
        _totalSupply = __totalSupply;
    }
    
    function balanceOf(address _owner) constant returns (uint balance)
    {
        balance = __balanceOf[_owner];
    }
    
    function transfer(address _to, uint _value) returns (bool success)
    {
        if (_value > 0 && __balanceOf[msg.sender] >= _value) {
            __balanceOf[msg.sender] -= _value;
            __balanceOf[_to] += _value;
            return true;
        }
        return false;
    }
    
    function transferFrom(address _from, address _to, uint _value) returns (bool success)
    {
        if (_value > 0 && __allowances[_from][msg.sender] >= _value 
            && __balanceOf[_from] >= _value) {
            __balanceOf[_from] -= _value;
            __balanceOf[_to] += _value;
            __allowances[_from][msg.sender] -= _value;
            return true;
        }
        return false;
    }
    
    function approve(address _spender, uint _value) returns (bool success)
    {   
        if (_value  > 0 && __balanceOf[msg.sender] >= _value) {
            __allowances[msg.sender][_spender] = _value;
            return true;
        }
        return false;
    }

    function allowance(address _owner, address _spender) constant returns (uint remaining)
    {
        return __allowances[_owner][_spender];
    }
}