function exitVaultFillingVaultInitiate(Hash.Order calldata o, uint256 a, Sig.Components calldata c) internal {
  bytes32 hash = validOrderHash(o, c);

  require(a <= (o.principal - filled[hash]), 'taker amount > available volume');
  
  filled[hash] += a;
      
  uint256 premiumFilled = (((a * 1e18) / o.principal) * o.premium) / 1e18;
  uint256 fee = ((premiumFilled * 1e18) / fenominator[3]) / 1e18;

  Erc20 uToken = Erc20(o.underlying);
  // transfer premium from maker to sender
  uToken.transferFrom(o.maker, msg.sender, premiumFilled);

  // transfer fee in underlying to swivel from sender
  uToken.transferFrom(msg.sender, address(this), fee);

  // transfer <a> vault.notional (nTokens) from sender to maker
  require(MarketPlace(marketPlace).p2pVaultExchange(o.underlying, o.maturity, msg.sender, o.maker, a), 'vault exchange failed');

  emit Exit(o.key, hash, o.maker, o.vault, o.exit, msg.sender, a, premiumFilled);
}
