(0) Obligation:

Generalized rewrite system (where rules with free variables on rhs are allowed):
The TRS R consists of the following rules:

U31(tt) → N
U41(tt) → s(plus(N, M))
and(tt) → X
U11(tt) → U12(isNat)
U12(tt) → U13(isNat)
U13(tt) → tt
U21(tt) → U22(isNat)
U22(tt) → tt
isNattt
isNatU11(and(isNatKind))
isNatU21(isNatKind)
isNatKindtt
isNatKindand(isNatKind)
isNatKindisNatKind
plus(N, 0) → U31(and(isNat))
plus(N, s(M)) → U41(and(and(isNat)))

(1) CritRuleProof (EQUIVALENT transformation)

The rule U31(tt) → N contains free variables in its right-hand side. Hence the TRS is not-terminating.

(2) NO