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

theorem
  i in dom f & i+j = len f + 1 implies f/.i = (Rev f)/.j
proof
  assume that
A1: i in dom f and
A2: i+j = len f + 1;
A3: j in dom Rev f by A1,A2,Th59;
A4: i = len f - j + 1 by A2;
  thus f/.i = f.i by A1,PARTFUN1:def 6
    .= (Rev f).j by A4,A3,Def3
    .= (Rev f)/.j by A3,PARTFUN1:def 6;
end;
