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 Th59:
  for f being FinSequence, i,j being Nat st
  i in dom f & i+j = len f + 1 holds j in dom Rev f
proof
  let f be FinSequence, i,j be Nat such that
A1: i in dom f and
A2: i+j = len f + 1;
A3: dom f = Seg len f & dom f = dom Rev f by Th57,FINSEQ_1:def 3;
  j = len f - i + 1 by A2;
  hence thesis by A1,A3,Th2;
end;
