
theorem
  for T being set, S being SetSequence of T st S is constant holds
  the_value_of S is Subset of T
proof
  let T be set, S be SetSequence of T;
  assume S is constant;
  then consider x being set such that
A1: x in dom S and
A2: the_value_of S = S.x by FUNCT_1:def 12;
  reconsider n = x as Element of NAT by A1;
  S.n in bool T;
  hence thesis by A2;
end;
