reserve D for non empty set;
reserve s for FinSequence of D;
reserve m,n for Element of NAT;

theorem Th20:
  for S be non empty set,
  x1,x2,x3,x4,x5,x6,x7,x8 be Element of S holds
  ex s be FinSequence of S st
  s is 8-element &
  s.1=x1&s.2=x2&s.3=x3&s.4=x4&
  s.5=x5&s.6=x6&s.7=x7&s.8=x8
  proof
    let S be non empty set,
    x1,x2,x3,x4,x5,x6,x7,x8 be Element of S;
    set a1=<*x1,x2,x3,x4*>;
    set a2=<*x5,x6,x7,x8*>;
    take b1 = a1^a2;
A1:  b1.1=a1.1 & ... & b1.4=a1.4 by FINSEQ_3:154;
    b1.(4+1)=a2.1 & ... & b1.(4+4)=a2.4 by FINSEQ_3:155;
    hence thesis by A1;
  end;
