Re: [chem] more on bonds and arrows

Dear Hans, Dear all

While browsing though the examples of `chem` Linux Debian provides in path
`/usr/share/doc/groff-base/examples/chem`, I noticed a chemical error in file
`ATP.chem`, i.e., a missing nitrogen atom.  This lead to an observation of
maybe a second one.

Given Hans' preliminary modifications of `chem` as shared in his email by
September 27th, I used the archived he included for a gradual correction
/ modification of `ATP.chem` in question.  To best of my knowledge, the new
bond types proposed were not yet added to the commit history of groff.  This
is the reason why this email includes a .zip archive the most relevant files
to ease a replication by Hans and other volunteers interested.

Revisions `atp_r03.chem` and above build on Hans' work (-> `Chem.pdf`), hence
can not be rendered by the current original `chem` preprocessor (->
`ChemO.pdf`).

@Hans, and interested volunteers:
Contrary to the examples compiled by Hans, I observe the proposed new bond
types `wedge` and `rwedge` work will within a plain cycle, e.g. `ring5` in
code block `atp_r03.chem:`.  They however appear to be incompatible with
an additional explicit definition of a non-C atom (`put O at 1`, block
`atp_r04.chem:` because `chem` now interprets them to belong to keyword
`double`.

This might represent a bug in Hans' proposition, separate from the missing
nitrogen atom in `ATP.chem` (last commit by 11 Novembre 2020).

@Branden:  My preference is to address the bond issue in `ATP.chem` first, and
subsequently consider filing a PR with a revised `ATP.chem` to the git history
of groff.

Kind regards,

Norwid