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Please help the idiot

In[] = D[p[x,t],x,t]/.{Derivative[n_,m_][p_][q__]->a^(n+m)}

Out[] = a^{2}

In[] = D[p[x,t],x,t]/.{Derivative[n__][p_][q__]->a^Plus[n]}

Out[] = a

WTF??? Why not a^{2}????

In the simplest case

In[] = D[p[x,t],x,t]/.{Derivative[n__][p_][q__]->Plus[n]}

Out []=Sequence[1, 1]

In[] = D[p[x,t],x,t]/.{Derivative[n__][p_][q__]->Plus[n,1]}

Out []= 3

Works!

In[] = D[p[x,t],x,t]/.{Derivative[n__][p_][q__]->(Plus[n,1]-1)}

Out []= Sequence[1, 1]

What hell is going on????

Thank you very much!

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# Please help with Rule in Mathematica

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