WARSAW (Reuters) - Poland s Deputy Foreign Minister Konrad Szymanski, commenting on Brexit proposals presented on Friday by British Prime Minister Theresa May, said that protecting the rights of European Union citizens in the United Kingdom was one of the key conditions.  We can agree to talk about a transitional period in relation between the EU and the UK only if that could lead to complete fulfillment of the UK s commitments toward the EU,  Szymanski, Poland s sherpa to the EU, said in emailed comments.  Another condition is also full protection of our citizens  rights in the UK. Membership of the UK in the single market as well as the customs  union translates into regulatory and financial obligations of the UK toward the EU.  