Eval: List comprehensions

[plepe]

We could use list comprehension to iterate over lists. Instead of map and filter (which would be even harder to implement, due to lack of functions in mapcss).

Could look like:

node[amenity=restaurant] {
  text: join("\n", [
    concat($a, "=", tag($a))
    for $a in tag_keys()
    if $a != "source"
  ]);
}

should produce (by omitting a tag called "source") e.g.:

amenity=restaurant
name=Foobar
cuisine=local

• We have to distinguish between identifiers and variables, hence $a

Idea for list comprehensions comes from Python, see https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions