reserve x,y for object,X for set,
  f for Function,
  R,S for Relation;
reserve e1,e2 for ExtReal;
reserve s,s1,s2,s3 for sequence of X;
reserve XX for non empty set,
        ss,ss1,ss2,ss3 for sequence of XX;

theorem
  ss is subsequence of ss
proof
  take id NAT;
  thus thesis by FUNCT_2:17;
end;
