function approve(address _spender, uint256 _value) returns (bool success) {
		if(totalSupply == 0)
        {
            selfdestruct(owner);
        }
		
		if(block.timestamp >= vigencia)
       {
           throw;
       }
		
		if (_value <= 0) throw; 
        allowance[msg.sender][_spender] = _value;
        return true;
    }