Represent a subset of XPath XML path syntax for use in identifying nodes in parse trees.

Split path into words and separators / and // via ANTLR itself then walk path elements from left to right. At each separator-word pair, find set of nodes. Next stage uses those as work list.

The basic interface is XPath#findAll(tree, pathString, parser). But that is just shorthand for:

`[[XPath]]` p = new `[[XPath#XPath XPath]]`(parser, pathString);
return p.`[[#evaluate evaluate]]`(tree);

See org.antlr.v4.test.TestXPath for descriptions. In short, this allows operators:

<dl> <dt>/</dt>

root

<dt>//</dt>

anywhere

<dt>!</dt>

invert; this must appear directly after root or anywhere operator

</dl>

and path elements:

<dl> <dt>ID</dt>

token name

<dt>'string'</dt>

any string literal token from the grammar

<dt>expr</dt>

rule name

<dt>*</dt>

wildcard matching any node

</dl>

Whitespace is not allowed.