reserve a,b,c,d,e for Real;
reserve X,Y for set,
          Z for non empty set,
          r for Real,
          s for ExtReal,
          A for Subset of REAL,
          f for real-valued Function;

theorem Th11:
  chi(X,Y) is Function of Y,REAL
  proof
    ({0,1} = {} implies X = {}) & {0,1} c= REAL by INT_1:3,NUMBERS:19;
    hence thesis by FUNCT_2:7;
  end;
