pragma solidity ^0.4.15;
import './ERC20.sol';
import './SafeMath.sol';
How connect SafeMath.sol from external(non-local) resourses?
This is probably what you mean:
pragma solidity ^0.4.0;
import "github.com/OpenZeppelin/zeppelin-solidity/contracts/math/SafeMath.sol";
contract MathExtended {
using SafeMath for uint;
function exec(uint a, uint b) returns (uint){
return a.add(b);
}
}
Solidity supports importing from Github directly, just remember not to include commits or branches when reference it must be the user/project/file-path/file.sol directly.
See http://solidity.readthedocs.io/en/develop/layout-of-source-files.html