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;

theorem Th15:
  base-{} = 0 & limit-{} = 0 & len-{} = 0
  proof
    not ex a st a in dom {};
    hence base-{} = 0 & limit-{} = 0 by Def4,Def5;
    hence thesis by ORDINAL3:56;
  end;
