theorem Th5:
  (SAT M).[n,'not' A]=1 iff (SAT M).[n,A]=0
 proof
  hereby assume(SAT M).[n,'not' A]=1;
   then A1: (SAT M).[n,A]=>(SAT M).[n,TFALSUM]=1 by Def11;
   (SAT M).[n,A]=1 or(SAT M).[n,A]=0 by XBOOLEAN:def 3;
   hence (SAT M).[n,A]=0 by A1,Def11;
  end;
  assume A2: (SAT M).[n,A]=0;
  thus(SAT M).[n,'not' A]=(SAT M).[n,A]=>(SAT M).[n,TFALSUM] by Def11
   .=1 by A2;
 end;
