YES
LCTRS
  Theories
    Core, Ints
  Signature
    sum: Int -> Int
  Rules
    sum(?4) -> +(?4, sum(+(?4, -1))) [not(>=(0, ?4))]
    sum(?5) -> 0 [>=(0, ?5)]
Confluent by Orthogonality with proof:
  no critical pairs

Elapsed Time:  34.95 ms