The List module is automatically loaded, but not assigned to a global
variable. It can be included in filters by calling `List = require
'List'`.
Lists of blocks, lists of inlines, and lists of classes are now given
`List` as a metatable, making working with them more convenient. E.g.,
it is now possible to concatenate lists of inlines using Lua's
concatenation operator `..` (requires at least one of the operants to
have `List` as a metatable):
function Emph (emph)
local s = {pandoc.Space(), pandoc.Str 'emphasized'}
return pandoc.Span(emph.content .. s)
end
Closes: #4081
