return return_result (false, cfg.err_msg_supl.form); -- fail if SBN has 'X' anywhere but last position
end  
return return_result (is_valid_isxn ('0' .. ident, 10), cfg.err_msg_supl.check);
end  
----------------------------< S S R N >----------------------------------------------------------------------  
Format an SSRN, do simple error checking  
SSRNs are sequential numbers beginning at 100? and counting up. This code checks the SSRN to see that it is
only digits and is greater than 99 and less than test_limit; the value in local variable test_limit will need
to be updated periodically as more SSRNs are issued.  
local function ssrn (options)
local id = options.id;
local handler = options.handler;
local id_num;
local text;  
id_num = id:match ('^%d+$'); -- id must be all digits  
if is_set (id_num) then -- id_num has a value so test it
id_num = tonumber (id_num); -- convert id_num to a number for range testing
if 100 > id_num or handler.id_limit < id_num then -- if SSRN is outside test limit boundaries
set_message ('err_bad_ssrn'); -- set an error message
options.coins_list_t['SSRN'] = nil; -- when error, unset so not included in COinS
end
else -- when id format incorrect
set_message ('err_bad_ssrn'); -- set an error message
options.coins_list_t['SSRN'] = nil; -- when error, unset so not included in COinS
end  
text = external_link_id ({link = handler.link, label = handler.label, q = handler.q, redirect = handler.redirect,
prefix = handler.prefix, id = id, separator = handler.separator, encode = handler.encode, access = options.access});  
return text;
end  
----------------------------< U S E N E T _ I D >------------------------------------------------------------  
Validate and format a usenet message id. Simple error checking, looks for 'id-@id-' not enclosed in
'<' and/or '>' angle brackets.  
local function usenet_id (options)
local id = options.id;
local handler = options.handler;  
local text = external_link_id ({link = handler.link, label = handler.label, q = handler.q, redirect = handler.redirect,
prefix = handler.prefix, id = id, separator = handler.separator, encode = handler.encode})  
if not id:match('^.+@.+$') or not id:match('^[^<].*[^>]$') then -- doesn't have '@' or has one or first or last character is '< or '>'
set_message ('err_bad_usenet_id') -- add an error message if the message id is invalid
options.coins_list_t['USENETID'] = nil; -- when error, unset so not included in COinS
end  
return text;
end  
----------------------------< Z B L >-----------------------------------------------------------------------  
A numerical identifier in the form nnnn.nnnnn - leading zeros in the first quartet optional  
format described here: http://emis.mi.sanu.ac.rs/ZMATH/zmath/en/help/search/  
temporary format is apparently eight digits. Anything else is an error  
local function zbl (options)
local id = options.id;
local handler = options.handler;  
if id:match('^%d%d%d%d%d%d%d%d$') then -- is this identifier using temporary format?
set_message ('maint_zbl'); -- yes, add maint cat
elseif not id:match('^%d?%d?%d?%d%.%d%d%d%d%d$') then -- not temporary, is it normal format?
set_message ('err_bad_zbl'); -- no, set an error message
options.coins_list_t['ZBL'] = nil; -- when error, unset so not included in COinS
end  
return external_link_id ({link = handler.link, label = handler.label, q = handler.q, redirect = handler.redirect,
prefix = handler.prefix, id = id, separator = handler.separator, encode = handler.encode});
end  
--============================<< I N T E R F A C E F U N C T I O N S >>==========================================  
--[[--------------------------< E X T R A C T _ I D S >------------------------------------------------------------  
Populates ID table from arguments using configuration settings. Loops through cfg.id_handlers and searches args for
any of the parameters listed in each cfg.id_handlers['...'].parameters. If found, adds the parameter and value to
the identifier list. Emits redundant error message if more than one alias exists in args  
local function extract_ids (args)
local id_list = {}; -- list of identifiers found in args
for k, v in pairs (cfg.id_handlers) do -- k is uppercase identifier name as index to cfg.id_handlers; e.g. cfg.id_handlers['ISBN'], v is a table
v = select_one (args, v.parameters, 'err_redundant_parameters' ); -- v.parameters is a table of aliases for k; here we pick one from args if present
if is_set (v) then id_list[k] = v; end -- if found in args, add identifier to our list
end
return id_list;
end  
--[[--------------------------< E X T R A C T _ I D _ A C C E S S _ L E V E L S >--------------------------------------  
Fetches custom id access levels from arguments using configuration settings. Parameters which have a predefined access
level (e.g. arxiv) do not use this function as they are directly rendered as free without using an additional parameter.  
returns a table of k/v pairs where k is same as the identifier's key in cfg.id_handlers and v is the assigned (valid) keyword  
access-level values must match the case used in cfg.keywords_lists['id-access'] (lowercase unless there is some special reason for something else)  
local function extract_id_access_levels (args, id_list)
local id_accesses_list = {};
for k, v in pairs (cfg.id_handlers) do
local access_param = v.custom_access; -- name of identifier's access-level parameter
if is_set (access_param) then
local access_level = args[access_param]; -- get the assigned value if there is one
if is_set (access_level) then
if not in_array (access_level, cfg.keywords_lists['id-access']) then -- exact match required
set_message ('err_invalid_param_val', {access_param, access_level});
access_level = nil; -- invalid so unset
end
if not is_set (id_list[k]) then -- identifier access-level must have a matching identifier
set_message ('err_param_access_requires_param', {k:lower}); -- parameter name is uppercase in cfg.id_handlers (k); lowercase for error message
end
id_accesses_list[k] = cfg.keywords_xlate[access_level]; -- get translated keyword
end
end
end
return id_accesses_list;
end  
----------------------------< B U I L D _ I D _ L I S T >----------------------------------------------------  
render the identifiers into a sorted sequence table  
<ID_list_coins_t> is a table of k/v pairs where k is same as key in cfg.id_handlers and v is the assigned value
<options_t> is a table of various k/v option pairs provided in the call to new_build_id_list;
modified by this function and passed to all identifier rendering functions
<access_levels_t> is a table of k/v pairs where k is same as key in cfg.id_handlers and v is the assigned value (if valid)  
returns a sequence table of sorted (by hkey - 'handler' key) rendered identifier strings  
local function build_id_list (ID_list_coins_t, options_t, access_levels_t)
local ID_list_t = {};
local accept;
local func_map = { --function map points to functions associated with hkey identifier
['ARXIV'] = arxiv,
['ASIN'] = asin,
['BIBCODE'] = bibcode,
['BIORXIV'] = biorxiv,
['CITESEERX'] = citeseerx,
['DOI'] = doi,
['EISSN'] = issn,
['HDL'] = hdl,
['ISBN'] = isbn,
['ISMN'] = ismn,
['ISSN'] = issn,
['JFM'] = jfm,
['JSTOR'] = jstor,
['LCCN'] = lccn,
['MR'] = mr,
['OCLC'] = oclc,
['OL'] = openlibrary,
['OSTI'] = osti,
['PMC'] = pmc,
['PMID'] = pmid,
['RFC'] = rfc,
['S2CID'] = s2cid,
['SBN'] = sbn,
['SSRN'] = ssrn,
['USENETID'] = usenet_id,
['ZBL'] = zbl,
}  
for hkey, v in pairs (ID_list_coins_t) do
v, accept = has_accept_as_written (v); -- remove accept-as-written markup if present; accept is boolean true when markup removed; false else
-- every function gets the options table with value v and accept boolean
options_t.hkey = hkey; -- ~/Configuration handler key
options_t.id = v; -- add that identifier value to the options table
options_t.accept = accept; -- add the accept boolean flag