(VAR)
(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 -> b6
  b5 -> b6
  c5 -> b6
)
(COMMENT
Example 13 {R_5} from [HM11]:
doi: http://dx.doi.org/10.1007/s10817-011-9238-x
)