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% push( ELEMENT, STACK, NEW )
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% True if NEW is [ELEMENT|STACK]
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push(ELEMENT,STACK,[ELEMENT|STACK]).
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% pop( STACK, TOP, NEW )
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% True if TOP and NEW are head and tail, respectively, of STACK
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pop([TOP|STACK],TOP,STACK).
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% empty( STACK )
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% True if STACK is empty
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empty([]).
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