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Accent escape sequences

It seems that there is no way to escape a single quote followed by a
space. I was looking at the accent escape sequences and realize that it
would be useful if these trigraphs were allowed to use a space as the
'letter' being modified. For example:

  "\' " becomes "'"
  "\~ " becomes "~"
  "\^ " becomes "^"
  "\% " becomes the degree symbol

Currently, there is no carat escape to avoid superscripts, and the
current tilde escape is only listed as "accepted by convention".

If you generalize the sequence <backslash><non-alphabetic><character> to
function like an old double-strike sequence, you can get other useful
combinations as well, for example "\/=" becomes not-equals.

I suspect that these trigraphs have not become better defined because
most people would rather just switch to some other modern encoding. But,
as an archival format, we are somewhat stuck with the current scheme,
and it probably makes sense to keep things in plain ASCII, and
human-readable. Also, I found another set of similar trigraph
definitions that are much more extensive at the bottom of the following
page:

http://abc.sourceforge.net/standard/abc2-draft.html

It is probably good to define a complete list of allowed trigraphs and
other codes, and do away with "accepted by convention" as a separate
list. I also think that it is worth extending the trigraphs to a more
complete set.

I am willing to try to make such a list if it is deemed useful, but
there are some things I already don't understand from the current set:

What is the purpose of \\rangle and \\langle; are these different from
"<" and ">"?

Why not use a more symbolic form for some items, like "\<-" instead of
"\\leftarrow"

Why do double and triple bond codes have names, and single bond is just
"---"?

Joe Krahn
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