Every now and then, the cultural biases inherent in LCSH (Library of Congress subject headings) pull me up short when trying to assign headings. The radical cataloguer Sanford Berman (yes, a radical cataloguer!) has made this field his own (and has even had some influence in changing some of them). However, there are many infelicities still lurking among the headings waiting to trap the unwary. Cataloguers are supposed to catalogue, not to pass judgment, but sometimes it is difficult to use the terms supplied without being judgmental.
The term which gave me pause for thought recently was "left-wing extremists". Unless I have missed something (which is quite possible, as I stumble my way through the dual barriers of American English and lack of natural language) this is the only generic term for people of left-wing political beliefs. You can have socialists and communists, and you can have "Right and left (Political science)", but none of these seemed quite right for a book about the left in Welsh politics - specifically, how the Labour Party copes with or reacts to or is affected by the left-wing element in Plaid Cymru. These are mainstream political parties, most of whose members would not describe themselves as extremists.
I muddled through without using the heading, pausing only to wonder whether in the eyes of Congress and its Library any person of left-leaning tendencies is automatically an "extremist".