Add blocks to access double-ended queues of bits

[stephanemagnenat]

Assuming we implement the FSM mode, the old state variables as in VPL1 are not useful for states any more. However, variables can be used for something else than states in the sense of program counter.

An idea to implement a variable model that is powerful and quite simple is the one of a binary double-ended queue. Assuming that VPL provides two binary queues and a block that can operate on them, many operations become possible, such as reading and comparing bar codes. The corresponding action block could provide the following operations:

• write 1/0 in the start/end of the queue
• pop from the start/end of the queue
• clear a queue

The corresponding event block could provide:

• compare the two queues lengths (same, smaller, larger)
• compare the two queues content (lower, smaller, larger), with empty values treated as zeros.