reserve A,B,C for Ordinal;
reserve a,b,c,d for natural Ordinal;
reserve l,m,n for natural Ordinal;
reserve i,j,k for Element of omega;
reserve x,y,z for Element of RAT+;

theorem Th40:
  {}/b = {} & a/1 = a
proof
A1: b <> {} implies RED(b,{}) = 1 by Th26;
  RED({},b) = {} by Th26;
  hence {}/b = {} by A1,Def10;
  RED(1,a) = 1 & RED(a,1) = a by Th24;
  hence thesis by Def10;
end;
