reserve i,j,k,n for Nat,
  D for non empty set,
  f, g for FinSequence of D;

theorem Th4:
  n in dom Rev f implies ex k st k in dom f & n+k = len f+1 & (Rev f)/.n = f/.k
proof
  assume n in dom Rev f;
  then n in dom f by FINSEQ_5:57;
  then consider k such that
A1: k in dom Rev f & n+k = len f+1 and
  f/.n = (Rev f)/.k by Th3;
A2: dom f = dom Rev f by FINSEQ_5:57;
  then (Rev f)/.n = f/.k by A1,FINSEQ_5:66;
  hence thesis by A1,A2;
end;
