reserve a,a1,a2,b,c,d for Ordinal,
  n,m,k for Nat,
  x,y,z,t,X,Y,Z for set;
reserve f,g for Function;
reserve A,B,C for array;

theorem
  f is (base-f)-based
  proof
    base-f = inf dom f by Th20;
    hence thesis by Th12;
  end;
