Before revision:  In order to increase the liquidity of the used hedging instruments for  currency options, the duality principle can be used to set up the hedges in a foreign market by using only European vanilla options sometimes along with a risk-less bond.
After revision:  In order to increase the liquidity of the used hedging instruments for currency options, the duality principle can be applied to set up the hedges in a foreign market by using only European vanilla options sometimes along with a risk-less bond.