Overview of surfaces
For various practical considerations, it sometimes happens that the
codes making up a text, written in a particular charset, cannot simply
be put out in a file one after another without creating problems or
breaking other things. Sometimes, 8-bit codes cannot be written on a
7-bit medium, variable length codes need kind of envelopes, newlines
require special treatment, etc. We sometimes have to apply "surfaces"
to a stream of codes, which surfaces are kind of tricks used to fit the
charset into those practical constraints. Moreover, similar surfaces
or tricks may be useful for many unrelated charsets, and many surfaces
can be used at once over a single charset.
So, `recode' has machinery to describe a combination of a charset
with surfaces used over it in a file. We would use the expression "pure
charset" for referring to a charset free of any surface, that is, the
conceptual association between integer codes and character intents.
It is not always clear if some transformation will yield a charset
or a surface, especially for those transformations which are only
meaningful over a single charset. The `recode' library is not overly
picky as identifying surfaces as such: when it is practical to consider
a specialised surface as if it were a charset, this is preferred, and
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