(VAR x)
(RULES
a1() -> b1()
a1() -> c1()
b1() -> b2()
c1() -> c2()
a2() -> b2()
a2() -> c2()
b2() -> b3()
c2() -> c3()
a3() -> b3()
a3() -> c3()
b3() -> b4()
c3() -> c4()
a4() -> b4()
a4() -> c4()
b4() -> b5()
c4() -> c5()
a5() -> b5()
a5() -> c5()
b5() -> d()
c5() -> d()
)
(COMMENT Vincent van Oostrom, RTA 2008)