Introduction - If you have any usage issues, please Google them yourself
		 
-- Mod-16 Counter using JK Flip-flops
-- Structural description of a 4-bit binary counter. 
-- The first two design entities describe a JK flip-flop and a 2-input AND gate respectively. 
-- These are then packaged together along with a signal named  tied_high  into a package named  jkpack . 
-- The counter design uses the package  jkpack , giving it access to the components and the signal declared within the package. 
-- The flip-flops and AND-gates are wired together to form a counter. 
-- Notice the use of the keyword OPEN to indicate an open-cct output port. 
-- some syntax can t be synthesized,it s for simulation only,such as "AFTER 5 ns"