What do $this->root->[hdom_info_end] mean, please?

Ask $this->root->_[hdom_info_end] what does it mean
When learning Simple_html_dom source code.
Found inside the grammar has such $this->root->_[hdom_info_end] = $this->cursor;

The new person to seek the expert advice, this what meaning ... Thank you.

------Solution--------------------
$this->root is a Simple_html_dom_node object (instantiated at 917 rows)
$this->root->_ is a property of the Simple_html_dom_node (defined at line 115)

The whole sentence is a value assignment.
What is the specific negative value, what is the function? To be determined by context
No, out of context.

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