Quoting Toralf Lund:
> What do I get if I do something like
>
> ${eval:$some_variable+1}
>
> when "some_variable" is not actually defined?
An undefined variable expands to nothing (except for acl_m/acl_c
variables, if you have the strict_acl_vars option set), so this is the
same as ${eval:+1}, which expands to "1".
Note that there was a change (which was reverted later) recently
regarding empty variables in numeric comparison operators (see the
changelog of 4.66).
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