If you run:
index( {"One", "Two", "Three", "Four"}, {100, 200, 300}, "Not Possible" )
you'd get, as expected:
{ "Not Possible", "Not Possible", "Not Posisble" }
However, if instead you run:
index( {"One", "Two", "Three", "Four"}, {100, 200, 300}, {} )
you'd get:
{ "One", "Two", "Three }
whereas I would have expected an empty list. Why does this happen?
Discussion posts and replies are publicly visible
Short answer appears to be that no one fully understands why.
Treat it as an undocumented feature?