anchor.diff:  
       // -- Self-referential operator functions --
   
  +  {$IFNDEF BIGINTEGERIMMUTABLE}
       /// <summary>
       ///   <para>The functional equivalent to</para>
       ///   <code>    A := A + Other;</code>
  
positive.code:   end;
  end;
  
  function BigInteger.Add(const Other: BigInteger): PBigInteger;
  var
    SelfSize, OtherSize: Integer;
  
negative.code: end;
  {$ENDIF}
  
  {$IFNDEF BIGINTEGERIMMUTABLE}
  function BigInteger.Multiply(const Other: BigInteger): PBigInteger;
  begin
    Result := @Self;
    Self := Self * Other;
  end;
  {$ENDIF}
  
  procedure FlipBigIntegerBit(var B: BigInteger; Index: Integer); inline;
  begin
  