### 2.5.4 New markup list command definition

Markup list commands are defined with the define-markup-list-command Scheme macro, which is similar to the define-markup-command macro described in New markup command definition, except that where the latter returns a single stencil, the former returns a list of stencils.

In a similar vein, interpret-markup-list is used instead of interpret-markup for converting a markup list into a list of stencils.

In the following example, a \paragraph markup list command is defined, which returns a list of justified lines, the first one being indented. The indent width is taken from the props argument.

#(define-markup-list-command (paragraph layout props args) (markup-list?)
#:properties ((par-indent 2))
(interpret-markup-list layout props
#{\markuplist \justified-lines { \hspace #par-indent #args } #}))


The version using just Scheme is more complex:

#(define-markup-list-command (paragraph layout props args) (markup-list?)
#:properties ((par-indent 2))
(interpret-markup-list layout props
(make-justified-lines-markup-list (cons (make-hspace-markup par-indent)
args))))


Besides the usual layout and props arguments, the paragraph markup list command takes a markup list argument, named args. The predicate for markup lists is markup-list?.

First, the function gets the indent width, a property here named par-indent, from the property list props. If the property is not found, the default value is 2. Then, a list of justified lines is made using the built-in markup list command \justified-lines, which is related to the make-justified-lines-markup-list function. A horizontal space is added at the beginning using \hspace (or the make-hspace-markup function). Finally, the markup list is interpreted using the interpret-markup-list function.

This new markup list command can be used as follows:

\markuplist {
\paragraph {
The art of music typography is called \italic {(plate) engraving.}
The term derives from the traditional process of music printing.
Just a few decades ago, sheet music was made by cutting and stamping
the music into a zinc or pewter plate in mirror image.
}
\override-lines #'(par-indent . 4) \paragraph {
The plate would be inked, the depressions caused by the cutting
and stamping would hold ink.  An image was formed by pressing paper
to the plate.  The stamping and cutting was completely done by
hand.
}
}


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