# Parentheses are optional for single key:
for (k1,           v in @a["b"]["c"]) { ... }
for ((k1),         v in @a["b"]["c"]) { ... }
# Parentheses are required for multiple keys:
for ((k1, k2),     v in @a["b"]["c"]) { ... } # Loop over subhashmap of a variable
for ((k1, k2, k3), v in @a["b"]["c"]) { ... } # Ditto
for ((k1, k2, k3), v in @a { ... }            # Loop over variable starting from basename
for ((k1, k2, k3), v in @* { ... }            # Loop over all variables (k1 is bound to basename)
