let t be Subset of (QuasiAdjs C); :: thesis: ex f being valuation of C st t at f c= t
consider f being empty valuation of C;
take f ; :: thesis: t at f c= t
let x be set ; :: according to TARSKI:def 3 :: thesis: ( not x in t at f or x in t )
assume x in t at f ; :: thesis: x in t
then ex a being quasi-adjective of C st
( x = a at f & a in t ) ;
hence x in t by ABCMIZ_1:139; :: thesis: verum