The first level is the string literal calculus D^String_Lit. For example, text files (left) and string literals (right) are both examples of document languages at this level:
left:
I'm suspicious of "strings".
right:
"I'm suspicious of \"strings\"."
Formally, D^String_Lit has no Computation and therefore the simplest semantics:
Expr^String_Lit 𝜀 ::= s Type^String_Lit 𝜏 ::= String
A string s ∈ String is a sequence of characters c ∈ Char, such as “x” or […]. We will present a sequence of document calculi D^String_. in the string domain.
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CRICHTON, Will and KRISHNAMURTHI, Shriram, 2024. A Core Calculus for Documents: Or, Lambda: The Ultimate Document. Proceedings of the ACM on Programming Languages. 5 January 2024. Vol. 8, no. POPL, p. 23:667-23:694. doi 
, p. 23:6.