let A be set ; :: thesis: for u being Element of (NormForm A)
for X being set st X c= u holds
X is Element of (NormForm A)
let u be Element of (NormForm A); :: thesis: for X being set st X c= u holds
X is Element of (NormForm A)
let X be set ; :: thesis: ( X c= u implies X is Element of (NormForm A) )
A1: u = @ u ;
assume X c= u ; :: thesis: X is Element of (NormForm A)
then X in Normal_forms_on A by A1, Th8;
hence X is Element of (NormForm A) by NORMFORM:def 14; :: thesis: verum