This is a Web patch file for pdfetex.
The change file creates an extra primitive named \tagcode, allowing read and write access to a character's char_tag info. The associated internal command is assign_font_int, with the new modifier value tag_code.
The only write operation that is (currently) performed is clearing the value of the char_tag, so it is more like `read/delete access. The operation is done by substracting a value based on the users input, using the following rules:
Nothing other that the char_tag value is changed, and nothing is saved. Changes are therefore irreversible. The primitive works directly on the font_info array, so all changes are \global, as for the other font assignment primitives.
If the character does not exist, the read operation returns -1. Otherwise, the return value is the 4 if ext_tag was set, 2 if list_tag was set, 1 if lig_tag was set, or 0 if the tag value was no_tag. The return values are chosen in such a way as to be consistent with the absolute value of the argument in the assigment case.
The \tagcode change file for pdfetex
A plain tex test file