[Retros] bug?

[pascal.wassong at free.fr]

Hi Paul,


> Dear retro friends,

> Natch offers the following proof game that is obviously illegal:

> 1.Pd2-d4   Pa7-a5   2.Ke1-d2   Ra8-a6   3.Kd2-c3   Ra6-c6+  4.Sb1-d2.

>

> Is this a bug?


No, this is not a bug, this is a feature. :-)

The stipulation of the position is wrong : you asked for a proof game where
white played the last move, but the white king is in check.  In such a case,
Natch finds solutions like the example you give.

This is due to the way Natch tests for checks : when a move can capture the
king, then Natch knows that the previous move was illegal.  In your example, a
black 4th move is missing, which would allow Natch to see that Sb1-d2 is
illegal.

I hope that my explanations are clear.  If not, please mail me privately (natch
at free dot fr), this may not interest most readers of this list.

Best wishes,
Pascal