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; MeTTa allows to use a separate knowledge base (Space) &kb
; to accumulate inferred knowledge
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; Bind &kb to a new empty Space
!(bind! &kb (new-space))

; Some knowledge
(= (frog $x)
   (and (croaks $x)
        (eat_flies $x)))
(= (croaks Fritz) True)
(= (eat_flies Fritz) True)
(= (croaks Sam) True)
(= (eat_flies Sam) True)
(= (green $x)
   (frog $x))

; Define conditional
(: ift (-> Bool Atom Atom))
(= (ift True $then) $then)

; For anything that is green, assert it is Green in &kb
; There should be two green things
!(assertEqualToResult
  (ift (green $x)
       (add-atom &kb (Green $x)))
  (() ()))

; Retrieve the inferred Green things: Fritz and Sam.
!(assertEqualToResult
  (match &kb (Green $x) $x)
  (Fritz Sam))