Example with Lisp Syntax

((prereq cse142 cse143))

((prereq cse143 cse373))

((prereq cse373 cse415))

; To prove X must come before Y, it’s

; enough to show that X is a prereq. for y.

((before X Y) (prereq X Y))

; Alternatively, one can show X must come

; before Y by showing that for some Z,

; X is a prereq of Z and Z comes before y.

((before X Y) (prereq X Z) (before Z Y))

; Before(x,y) v ~Prereq(x,z) v ~Before(z,y)

; ( <head> <-- <atomic formula 1> <af2> ...<afn>)