Prevertical format validator

Bash script available here: scripts/prevertical-validator.sh

The validator wraps the prevertical data by adding:

an XML header,
an XML DTD,
a root element <corpus/> and sends all to xmllint which is an easy to use XML parser and validator.

The main requirements for validity are:

root element,
document elements <doc/> and paragraph elements <p/>,
for structure attributes: & escaped to &, < escaped to <, " escaped to &quot;,
and for text nodes: & escaped to & and < escaped to <,
no empty structures (incl. singleton tags),
space sequences are normalized to single ordinary blanks,
all paragraph text is on a single line.

The DTD is there:

to make sure all structures are well nested, e.g. p within doc and not vice versa, no p outside of doc, etc.
to check all required attributes are there.