reserve a,b,c,d,m,x,n,j,k,l for Nat,
  t,u,v,z for Integer,
  f,F for FinSequence of NAT;
reserve p,q,r,s for real number;
reserve a,b,c,d,m,x,n,k,l for Nat,
  t,z for Integer,
  f,F,G for FinSequence of REAL;
reserve q,r,s for real number;
reserve D for set;

theorem Th9:
  n in dom f implies (f|n).1 = f.1
  proof
    assume
    A1: n in dom f; then
    n >= 1 by FINSEQ_3:24,NAT_1:14;
    hence thesis by A1,Th7;
  end;
