All the deponent verbs I've run across so far had an ο for theme vowel, as in: βούλομαι or φαίνομαι. Contraction resulted in an ω (as in θεᾰ́ομαι to θεῶμαι) or ου (ἡγέομαι to ἡγοῦμαι).
In general, I have not seen ο "losing" to another vowel (e.g. α or ε) in a combination and disappearing. For example, α + ο = ω, ε + ο = ου, and ο + ε = ου. To wit, ο survives.
I thought of two possibilities.
κεῖμαι never had an ο. For example, it comes from κέεμαι.
κεῖμαι came by a strange sort of contraction from κέομαι.
Please tell me what is going on, and if possible provide other verbs that behave like κεῖμαι.
I believe a similar question arises in respect of ὑποκείμενος. So far I have only seen -μενος with α or ο for a theme vowel (as in πεμψάμενος or πεμπόμενος) or without a theme vowel (as in πεπεμμένος). Contraction might give me ου (as in οἰκούμενος). But how do we end up with ει in ὑποκείμενος? Thanks.