(VAR x y z)
(RULES
or(true,y) -> true
or(x,true) -> true
or(false,false) -> false
mem(x,nil) -> false
mem(x,set(y)) -> =(x,y)
mem(x,union(y,z)) -> or(mem(x,y),mem(x,z))
)

(COMMENT Example 2.40 (Member) in \cite{SK90})