def addUnit(newUnit, newRate)

			if @dictionary.nil?
				@dictionary = Hash.new()
			else
				# do nothing
			end

			# certify if the key doesn't exist
			if !@dictionary.has_key?(newUnit)
				@dictionary[newUnit] = newRate

				# verify if the key has been added
				if @dictionary.has_key?(newUnit)
					puts "key #{newUnit} added"

					# return @dictionary
					return true
				else
					# throw exception
					return false
				end

			else
				puts "key #{newUnit} already exists"
				return false
			end

		end
def addUnit(newUnit, newRate)

			if @dictionary.nil?
				@dictionary = Hash.new()
			else
				# do nothing
			end

			# certify if the key doesn't exist
			if !@dictionary.has_key?(newUnit)
				@dictionary[newUnit] = newRate

				# verify if the key has been added
				if @dictionary.has_key?(newUnit)
					puts "key #{newUnit} added"

					# return @dictionary
					return true
				else
					# throw exception
					return false
				end

			else
				puts "key #{newUnit} already exists"
				return false
			end

		end
newUnit: name of the new unit to be added
 newRate: reason between new unit and base unit
 (example: kilometer it's 1000x greater than meter, so the newRate should be 1000)
 returns bool - true if succeed, false if fails