Sets the priority of a. Valid priorities are start at 0 and go to one less than gtk_text_tag_table_get_size(). Each tag in a table has a unique priority; setting the priority of one tag shifts the priorities of all the other tags in the table to maintain a unique priority for each tag. Higher priority tags "win" if two tags both set the same text attribute. When adding a tag to a tag table, it will be assigned the highest priority in the table by default; so normally the precedence of a set of tags is the order in which they were added to the table, or created with gtk_text_buffer_create_tag(), which adds the tag to the buffer's table automatically.