
(a?[r]>r![] & a?[r]>r![] & a?[r]>r![])
	& a![b] & (b?[]>nil)
	& a![c] & (c?[]>nil) ===> X.

eg
	a![c]
=> (c![]&a?[c]>c![]&a?[c]>c![])&a![b]&(b?[]>nil)&nil&c?[]>nil
	c![]
=> (nil&a?[c]>c![]&a?[c]>c![])&a![b]&(b?[]>nil)&nil&nil
	a![b]
=> (nil&b![]&a?[b]>b![])&nil&(b?[]>nil)&nil&nil
	b![]
=> (nil&nil&a?[b]>b![])&nil&nil&nil&nil

