See issue #1175
BTW, we may have to revise this decision in the future when we decide to
populate the string library with lemmas.
It is inconvenient to prove the lemmas at string/basic.lean since the
tactic framework has not been defined yet.
Anyway, I think it is worth to keep the private for now, and make sure
nobody relies on its implementation.
We want to make sure string users do not depend on the string
implementation. This is the first step.
We need this refactoring *now* to make sure it will not be
super painful to address issue #1175
This suggestion has been discussed at Slack.
We have decided to use #"c" as notation because we wanted to allow `'`
in the beginning of identifiers like in SML and F*. In particular,
we wanted to allow users to use 'a 'b 'c for naming type parameters
like in SML. However, nobody used this notation. In the Lean standard
library, we are using greek letters for naming type parameters.
So, there is no real motivation for the ugly #"c" syntax.
