For a long time, "string.find (s, pattern [, init [, plain]])"
"Note that if plain is given, then init must be given as well." This
is a bit surprising since a lot of other functions, like
[table.]unpack, or load, or utf8.* ones, have several optional
arguments but don't have similar notices; one would think that it says
init must be non-nil but init can be supplied as nil quite all right
in such cases: