Just means that just as you could write mathematical statements in programmatic

Just means that just as you could write mathematical statements in programmatic or ordinary language form, you can also convert that ordinary langauge back to programmatic and mathematic form.

Silly example:
“The cat’s black”
-vs-
“I promise is see a cat and that the fur on the cat reflects the color we call black, and if you also observe the same cat you will agree that you also see the fur on the cat reflects the color we call black.”

Promissory testimony, in operational terms, absent the copula to be, in a complete transaction.

So the statement is Expandable into a complete sentence (transaction) AND Reducible into a simple statement (declaration).