reserve p,q,r,s for boolean object;

theorem
  (p => q) => ('not' (q '&' r) => 'not' (p '&' r)) = TRUE
proof
A1: q = FALSE or q = TRUE by Def3;
A2: p = FALSE or p = TRUE by Def3;
  r = FALSE or r = TRUE by Def3;
  hence thesis by A1,A2;
end;
