# Square filter

A*square filter*consists of the keyword

`square`

, then an optional keyword `all`

, then a variable,
then the keyword `in`

, then a set filter, then an arbitrary filter:
squarevariableinin_filterbodysquare allvariableinin_filterbody

Here, *variable* is a set variable; *in_filter* is a set filter; and *body* is any filter.

For example,

square x in R x attacks ksquare z in a1-8 a attacks z

square all z in A power z<2

In the first example above, *variable* is `x`

; *in_filter* is `R`

; and *body* is `x attacks k`

.

The operation of the square filter when the the optional `all`

parameter is absent is as follows:

- The set
*in*represented by*in_filter*in the current position is computed - For each square
*s*in*in*,- the variable
*x*is set to*s*. - (*)
*body*is evaluated in the current position

- the variable
- The
`square`

filter represents the set of*s*in*in*for which*body*was true in step (*)

The operation of the square filter when the `all`

parameter *is* present is similar:

- The set
*in*represented by*in_filter*in the current position is computed - For each square
*s*in*in*,- the variable
*x*is set to*s*. - (*)
*body*is evaluated in the current position

- the variable
- The
`square`

filter matches the position if*body*is never false in step (*): for each*s*in*in*, the*body*is true

Thus, a `square`

filter is a set filter only if the `all`

parameter is absent.

## Example of a square filter

Suppose the current position is set to the following position:
Now let's compute what happens when the following `square`

filter is evaluted on that position:

square x in h1-8 R attacks x x attacks K

To determine the set of squares represented by this square filter in the diagram, the variable `x`

will be successively set to each square in the
set represented by the following the word `in`

, namely `h1-8`

.

The square filter will match an assignment of `x`

to a particular square only if a white rook attacks that square and if that square attacks the white King.
In the current position, note that:

`qh3`

attacks the white king's square and is attacked by a white rook - match!
`nh6`

attacks the white king's square but is NOT attacked by the white rook - no match
`Nh4`

attacks the white king's square but is NOT attacked by the white rook - no match
`Bh7`

attacks the white king's square and is attacked by Ra7 - match!

all other squares on the h-file fail both clauses
The result is that filter matches when `x`

is either `h3`

or `h7`

.

Therefore, the square filter represents the
set of squares consisting of `h3`

and `h7`

. In particular, the square filter will match the current position.