theorem Th12:
  S-min C in C & S-max C in C
proof
A1: S-min C in S-most C by PSCOMP_1:58;
A2: S-max C in S-most C by PSCOMP_1:58;
  S-most C c= C by XBOOLE_1:17;
  hence thesis by A1,A2;
end;
