The interpreter now has an "underscore is last value" flag, which makes
Interpreter::get_variable() return the last value if:
- The m_underscore_is_last_value flag is enabled
- The name of the variable lookup is "_"
- The result of that lookup is an empty value
That means "_" can still be used as a regular variable and will stop
doing its magic once anything is assigned to it.
Example REPL session:
> 1
1
> _ + _
2
> _ + _
4
> _ = "foo"
"foo"
> 1
1
> _
"foo"
> delete _
true
> 1
1
> _
1
>
5072d4e02d