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Metamath Proof Explorer

Theorem nnzi

Description: A positive integer is an integer. (Contributed by Mario Carneiro, 18-Feb-2014)

Ref Expression
Hypothesis nnzi.1 
|- N e. NN
Assertion nnzi 
|- N e. ZZ

Proof

Step Hyp Ref Expression
1 nnzi.1 
 |-  N e. NN
2 nnssz 
 |-  NN C_ ZZ
3 2 1 sselii 
 |-  N e. ZZ