Metamath Proof Explorer


Theorem nfich1

Description: The first interchangeable setvar variable is not free. (Contributed by AV, 21-Aug-2023)

Ref Expression
Assertion nfich1 xxyφ

Proof

Step Hyp Ref Expression
1 df-ich xyφxyxayxayφφ
2 nfa1 xxyxayxayφφ
3 1 2 nfxfr xxyφ