Lua filters: iterate over AST element fields when using pairs

This makes it possible to iterate over all field names of an AST element
by using a generic `for` loop with `pairs`:

    for field_name, field_content in pairs(element) do
      …
    end

Raw table fields of AST elements should be considered an implementation
detail and might change in the future. Accessing element properties
should always happen through the fields listed in the Lua filter docs.

Note that the iterator currently excludes the `t`/`tag` field.

data/pandoc.lua

@@ -73,6 +73,39 @@ local function create_accessor_functions (fn_template, accessors)
   return res
 end
 
+--- Get list of top-level fields from field descriptor table.
+-- E.g.: `top_level_fields{'foo', {bar='baz'}, {'qux', 'quux'}}`
+-- gives {'foo, 'bar', 'qux', 'quux'}
+-- @local
+local function top_level_fields (fields)
+  local result = List:new{}
+  for _, v in ipairs(fields) do
+    if type(v) == 'string' then
+      table.insert(result, v)
+    elseif type(v) == 'table' and #v == 0 and next(v) then
+      table.insert(result, (next(v)))
+    else
+      result:extend(top_level_fields(v))
+    end
+  end
+  return result
+end
+
+--- Creates a function which behaves like next, but respects field names.
+-- @local
+local function make_next_function (fields)
+  local field_indices = {}
+  for i, f in ipairs(fields) do
+    field_indices[f] = i
+  end
+
+  return function (t, field)
+    local raw_idx = field == nil and 0 or field_indices[field]
+    local next_field = fields[raw_idx + 1]
+    return next_field, t[next_field]
+  end
+end
+
 --- Create a new table which allows to access numerical indices via accessor
 -- functions.
 -- @local
@@ -102,6 +135,15 @@ local function create_accessor_behavior (tag, accessors)
       rawset(t, k, v)
     end
   end
+  behavior.__pairs = function (t)
+    if accessors == nil then
+      return next, t
+    end
+    local iterable_fields = type(accessors) == 'string'
+      and {accessors}
+      or top_level_fields(accessors)
+    return make_next_function(iterable_fields), t
+  end
   return behavior
 end
 
@@ -842,6 +884,14 @@ M.Attr.behavior.__newindex = function(t, k, v)
     rawset(t, k, v)
   end
 end
+M.Attr.behavior.__pairs = function(t)
+  local field_names = M.Attr.behavior._field_names
+  local fields = {}
+  for name, i in pairs(field_names) do
+    fields[i] = name
+  end
+  return make_next_function(fields), t, nil
+end
 
 -- Citation
 M.Citation = AstElement:make_subtype'Citation'
@@ -892,6 +942,14 @@ M.ListAttributes.behavior.__newindex = function (t, k, v)
     rawset(t, k, v)
   end
 end
+M.ListAttributes.behavior.__pairs = function(t)
+  local field_names = M.ListAttributes.behavior._field_names
+  local fields = {}
+  for name, i in pairs(field_names) do
+    fields[i] = name
+  end
+  return make_next_function(fields), t, nil
+end
 
 
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