gistfile1.txt

contract Vault {
  Gem public top;
  Gem public bot;
  constructor(address _top, address _bot) {
    top = _top; bot = _bot;
  }

  function cost(uint amt) external returns (uint);
  function exit(uint amt) external returns (uint);
  function join(uint amt) external returns (uint out) {
     // #VARIANT burn and mint
     out = cost(amt);
     top.burn(msg.sender, amt);
     bot.mint(msg.sender, out);
     return out;

     // #VARIANT hold and mint
     out = cost(amt);
     top.pull(msg.sender, amt);
     bot.mint(msg.sender, out);
     return out;
     
     // etc
  }
}

contract VaultWrap {
  GemFab public gf;
  mapping(address=>bool) public built;
  constructor (GemFab _gf) {
    gf = _gf;
  }
  function wrap(address bot, string name, string symbol) returns (Vault) {
    require(gf.built(bot), 'ERR_FAKE_GEM');
    address top = gf.build(name, symbol);
    address vault = new Vault(top, bot);
    built[vault] = true;
    return vault;
  }
}