Logical inconsistency in the paragraph about copula past tense

[Konstantin-Glukhov]

Section 2.3.22 - Past tense
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The common explanation for だ is that it came from で, the continuative of です, and the verb ある, to form the copula である. This copula is actually still used in modern Japanese in formal settings. However, the で+あ in this である has contracted over time, to form だる, which explains the 未然形 for だ, which is だろ.

1. The first highlighted statement is inconsistent with でし (連用形 of です introduced earlier in section 2.3.2)
2. The second highlighted text presents a statement with implied premises not obvious to the reader. Since 未然形 of ある is あら it is not obvious how だろ sprang from だる. Detailed explanation of だろ etymology will dramatically improve the clarity of the paragraph.

[pkmkb005]

Waiting for this issue to get solved. Any progress @Pomax ?