public void trade(ItemStack item, int cost)
    {
        int totalErium = 0;
        for (ItemStack stack: theplayer.inventory.mainInventory)
            if (stack != null && stack.itemID == mod_ArcticCraft.eriumStone.shiftedIndex)
                totalErium += stack.stackSize;

        if (totalErium < cost)
        {
            theplayer.addChatMessage("You don't have enough Erium to make this trade.");
            return;
        }

        // removes the stuff from the inventory.
        for (int i = 0; i < theplayer.inventory.mainInventory.length; i++)
        {
            ItemStack stack = theplayer.inventory.mainInventory[i];
            if (stack != null && stack.itemID == mod_ArcticCraft.eriumStone.shiftedIndex)
            {
                if (stack.stackSize >= cost)
                {
                    stack.stackSize -= cost;
                    if (stack.stackSize == 0)
                        theplayer.inventory.mainInventory[i] = null;
                    break; // or return, and and the method.
                }

                else if (stack.stackSize < cost)
                {
                    cost -= stack.stackSize;
                    theplayer.inventory.mainInventory[i] = null;
                }
            }
        }

        theplayer.inventory.addItemStackToInventory(item);
    }