Proto-Celtic origins of Welsh and Breton forms of the verb “to be”?

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Hello, I’m trying to understand the reconstructed Proto-Celtic forms behind the Welsh and Breton paradigms of the verb "to be"

As far as I understand:

Welsh yw and Breton eo may derive from Proto-Celtic *ets

Welsh mae / Breton emañ might come from something like *ets mendu-

Welsh sydd / Breton zo has been linked to a possible *etsi io formation

However, I’m not confident about these reconstructions or how widely accepted they are.

I’m also unsure about the origins of Welsh ydw and Breton eus.

In addition, Old Breton attests forms like is for “to be,” which may also go back to ets, but I’m having trouble connecting all these developments into a coherent picture.

Could anyone clarify the Proto-Celtic reconstructions involved, or point me toward reliable explanations of how these forms developed?