reserve k,n for Nat,
  x,y,z,y1,y2 for object,X,Y for set,
  f,g for Function;
reserve p,q,r,s,t for XFinSequence;
reserve D for set;

theorem Th33:
  (p^<%x%>).(len p)=x
proof
A1: dom <%x%> = 1 & 0 in Segm(0+1) by Def4,NAT_1:45;
  len p + 0 = len p;
  hence (p^<%x%>).(len p) = <%x%>.0 by A1,Def3
    .=x;
end;
