Xy-pic User's Guide

## 1.5 Breaks

It is also possible to "break" an arrow with a label using the `|` character: `$\xymatrix@1{A\ar[r]``|``f&B}$` will set .

If you just want an empty break you should use the special `\hole` break: the arrow was typeset by including `$\xymatrix@1{ A\ar[r]``|``\hole & B }$` in the text.

A different use of breaks is to place a label somewhere in a diagram outside the normal matrix mesh: this is accomplished by "breaking" an invisible arrow obtained using the `@{}` arrow style: the square

was typeset by

>     \xymatrix{\ar @{} [dr] |{=}
>      A \ar[d] \ar[r] & B \ar[d] \\
>      B \ar[r]        & C        }

There is more on breaks in section 2.7.

#### Exercise 4.

Typeset the first axiom of category theory as the display

Answer.

Xy-pic User's Guide