change return value of
duplicable? with Ruby 2.4+
As described at the top of the file, this method exists to avoid the expensive rescue.
We should use the dup+rescue pattern at method definition time, instead -- something similar to 440559f.
@sgrif this seems to leave
I think this isn't as simple as this for symbols.
Somehow some symbols are duplicable, but others not.
I can't work out why