reserve A,B,C for Ordinal,
  K,L,M,N for Cardinal,
  x,y,y1,y2,z,u for object,X,Y,Z,Z1,Z2 for set,
  n for Nat,
  f,f1,g,h for Function,
  Q,R for Relation;
reserve ff for Cardinal-Function;
reserve F,G for Cardinal-Function;
reserve A,B for set;

theorem Th50:
  {} in sproduct f
proof
A1: dom {} c= dom f;
  for x being object st x in dom {} holds {} .x in f.x;
  hence thesis by A1,Def9;
end;
