reserve E for compact non vertical non horizontal Subset of TOP-REAL 2,
  C for compact connected non vertical non horizontal Subset of TOP-REAL 2,
  G for Go-board,
  i, j, m, n for Nat,
  p for Point of TOP-REAL 2;

theorem
  for D being non empty set, f being FinSequence of D for i,j st 1 <= i
  & i <= len f & 1 <= j & j <= len f holds mid(f,i,j) is non empty
proof
  let D be non empty set, f be FinSequence of D;
  let i,j;
  assume that
A1: 1 <= i and
A2: i <= len f and
A3: 1 <= j and
A4: j <= len f;
A5: j in dom f by A3,A4,FINSEQ_3:25;
  i in dom f by A1,A2,FINSEQ_3:25;
  hence thesis by A5,SPRECT_2:7;
end;
