I'm reading the documentation Metaprogramming <http://docs.julialang.org/en/latest/manual/metaprogramming/> chapter. To me, the `$` operator is different from the one in the string interpolation. In the string, `$` is a runtime interpolation. In the expression, `$` is a compile-time interpolation.
In one example provided, `ex = :(a in $:((1,2,3)) )`, I find that it's equal to `ex = :(a in (1,2,3) )`. So I do not see the intention of `$`. In the next example, there is `:a + :b` in the expression, but the sum of `Symbols` is not defined. Is it wrong?
