I'm trying to understand why Bash removes double quotes (but not single quotes) when doing variable expansion with `${parameter:+word}` (*Use Alternate Value*), in a here-document, for example:

```
% var=1
% cat <<EOF
> ${var:+"Hi there"}
> ${var:+'Bye'}
> EOF
Hi there
'Bye'

```

According to the manual, the "word" after `:+` is processed with tilde expansion, parameter expansion, command substitution, and arithmetic expansion. None of these should do anything.

What am I missing? How can I get double quotes in the expansion?