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By default, external processing instructions are hidden (for example, processing instructions used to store metadata in other XML editors). If you want them to be displayed (for example, to edit them), they must be associated with the current document using a CSS like this:
@namespace oxy "http://www.oxygenxml.com/extensions/author";
oxy|processing-instruction[Pub],
oxy|processing-instruction[PubTbl],
oxy|processing-instruction[xm-replace_text],
oxy|processing-instruction[xm-deletion_mark],
oxy|processing-instruction[xm-insertion_mark_start],
oxy|processing-instruction[xm-insertion_mark_end]
{
    display:block !important;
}