theorem
  s is constant & s1 is subsequence of s implies s = s1
proof
  assume that
A1: s is constant and
A2: s1 is subsequence of s;
  let n be Element of NAT;
  consider N such that
A3: s1=s*N by A2,Def13;
  thus s1.n=s.(N.n) by A3,FUNCT_2:15
    .=s.n by A1,Th23;
end;
