• If Qb3 is insufficient,
  there must be another queen that can move on b3.
• If Qab3 is insufficient,
  there must be another queen in column a that can move on b3.
• If Q2b3 is insufficient,
  there must be another queen in rank 2 that can move on b3.
• Since both Qab3 and Q2b3 are insufficient, the board must be crawling with queens poised to attack b3.

As a side note:

Qb3 should give an error if

There are more than one Q on the board and they can move to b3.

That will be an interesting check!

I assumed dragonchild meant what s/he said :).

 

I said what I meant and meant what I said. I even gave an example.

  +---+---+---+---+---+---+---+---+
8 |   |   |   |   |   |   |   |   | 
  +---+---+---+---+---+---+---+---+
7 |   |   |   |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
6 |   |   |   |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
5 |   |   |   |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
4 |   |   |   |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
3 | Q | x |   |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
2 | Q |   | Q |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
1 |   |   |   |   |   |   |   |   |
  +---+---+---+---+---+---+---+---+
    a   b   c   d   e   f   g   h
[download]

How would you indicate that the queen at a2 moves to b3? Q2b3 doesn't work, and Qab3 doesn't work. You have to say Qa2b3. And, before you say this is far-fetched, the notation isn't for normal games. It has to describe every single possible game.

Update: It seems that your link to wikipedia even describes this situation, and I quote:

It may be necessary to identify a departing piece with both its file and its rank in unusual configurations (e.g. the player has 3 queens or 3 knights on the board).

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We are the carpenters and bricklayers of the Information Age.

Then there are Damian modules.... *sigh* ... that's not about being less-lazy -- that's about being on some really good drugs -- you know, there is no spoon. - flyingmoose

I shouldn't have to say this, but any code, unless otherwise stated, is untested

People use and generate chess notation and they will enjoy being able to follow their usual habits, especially if they are the sort to fall into english descriptive or long algebraic under stress.

Any good chess notation parser will need to maintain or reference a board image.