def assert_currency! arg
      defines?(arg) ? (country_map[arg] || arg) : raise(Exchange::NoCurrencyError.new("#{arg} is not a currency nor a country code matchable to a currency"))
    end
def assert_currency! arg
      defines?(arg) ? (country_map[arg] || arg) : raise(Exchange::NoCurrencyError.new("#{arg} is not a currency nor a country code matchable to a currency"))
    end
Asserts a given argument is a currency. Tries to match with a country code if the argument is not a currency
 @param [Symbol, String] arg The argument to assert
 @return [Symbol] The matching currency as a symbol