Returns a list containing all keys in a util/map/hashmap.
Since: 3.0
Parameters
map: The map.
Examples
use <util/map/hashmap.scad>
use <util/map/hashmap_keys.scad>
m = hashmap([["k1", 10], ["k2", 20], ["k3", 30]]);
echo(hashmap_keys(m)); // a list contains "k1", "k2", "k3"