# `∪`

filter

The ` ∪ `

filter has the syntax:
left∪right

where

and *left*

are each filters. The *right*

and *left*

arguments must each be set filters.
*right*

The `∪`

filter is a set filter whose
value is the union of the sets of squares represented by its arguments.

For example,

pin ∪ {♖→♚}

is the set of squares on which there is either a pinned
piece or there a white rook attacking the black king.
The

argument to the *left*`∪`

filter is pin whose value is the set of squares on which there is a pinned piece. The

argument to the *right*`∪`

filter is `{♖→♚}`

, whose value is the set of squares
on which there is a white rook which attacks the black king. The value of the `∪`

filter is the union of these two sets.

# Examples

The`∪`

filter is used throughout the examples.