Understanding :<= vs :<<= etc.

I am trying to understand the compiler directives:



:=> v/s :=>> or the reverse :<<= vs :<=. Also, <=> vs <<=> OR <=>>.



1. Take the inital code:



(defconcept person (?x))

(defrelation alive (?x))



(defconcept live-person (?x person)

  :<=> (alive ?x)

  )



Print out the internal form: (print-rules live-person)



(FORALL (?x1)

   (<= (PERSON ?x1)

       (LIVE-PERSON ?x1))) ; sub-concept rule, ignore



(FORALL (?x1)

   (<= (ALIVE ?x1)

       (LIVE-PERSON ?x1)))   ;; compare with the second version below



(FORALL (?x1)

   (<= (LIVE-PERSON ?x1)     ;; compare with third version below

       (ALIVE ?x1)))



2. Change the definition of live-person to use the compiler directive :forward-only

(defconcept live-person (?x person)


  :<=>> (alive ?x)


  )



Print out the internal form: (print-rules live-person)



(FORALL (?x1)

   (<= (PERSON ?x1)

       (LIVE-PERSON ?x1)))    ; sub-concept rule, ignore



(FORALL (?x1)

   (=>> (LIVE-PERSON ?x1)    ; only difference from previous form

        (ALIVE ?x1)))



(FORALL (?x1)

   (<= (LIVE-PERSON ?x1)      ; same as before

       (ALIVE ?x1)))  



I guess the difference is that, in the second version 

-- alive can be deduced if liveperson is given as a fact but not the reverse

-- When live-person is given as a fact, alive is also asserted via
forward-chaining. this does not happen in the first, which has to find
alive at query time.

Am I correct?





3. Change the definition of live-person to use the compiler directive :backward-only


(defconcept live-person (?x person)



  :<<=> (alive ?x)



  )



Now see the expansion: (print-rules live-person)



(FORALL (?x1)

   (<= (PERSON ?x1)

       (LIVE-PERSON ?x1)))   ; sub-concept rule, ignore



(FORALL (?x1)

   (<= (ALIVE ?x1)

       (LIVE-PERSON ?x1)))   ; same as first version



(FORALL (?x1)

   (<<= (LIVE-PERSON ?x1)

        (ALIVE ?x1)))



What is the difference between the highlighted rule above and the one in the very first version:



(FORALL (?x1)


   (<= (LIVE-PERSON ?x1)


       (ALIVE ?x1)))



Thanks



Srini.

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