Reverting (though belatedly) to the discussion of the compound word
citta-gu, I would like to clarify why I said citta in this word
is not a past participle. It is always helpful to look at the
corresponding Sanskrit word to understand how a Pali word is formed.
(They say Sanskrit is a transparent language: you can often clearly
see the different elements that go to make up a word prefixes,
root, suffixes etc.) The Skt equivalent of Pali citta is citra. The
two elements are cit- and -ra. The suffix is ra, which is not used
to form past participles. PPs are formed with the suffixes ta and -
na, in Skt as in Pali. Skt tr- almost always appears in the
corresponding Pali word as tt-. So Skt citra becomes Pali citta.
What we must note is the history of the form. It is cit-ra which has
become citta. If you refer up a good Vedic or Sanskrit grammar like
A.A. Macdonell's, you will find ra listed as a "primary nominal
suffix" . Some other words with this suffix are ugra (Pali ugga),
grdhra (P. gijjha), k.sura (P. khura), abhra (P, abbha) etc.
Macdonnel in fact says that many adjectives are formed with this
suffix.
By the way, you can look up Skt citra in the excellent 2006
version of Monier Williams Sanskrit Dictionary published online by
the University of Cologne.