DSL EXPRESSION:
for ((k1,k2,k3), v in @*) {}

AST:
"block" [tt:block] [nt:StatementBlock]
    "for" [tt:kw_for] [nt:ForLoopMultivariable]
        "multi_index" [tt:multi_index] [nt:MultiIndex]
            "k1" [tt:non_sigil_name] [nt:LocalVariable]
            "k2" [tt:non_sigil_name] [nt:LocalVariable]
            "k3" [tt:non_sigil_name] [nt:LocalVariable]
        "v" [tt:non_sigil_name] [nt:LocalVariable]
        "@*" [tt:full_oosvar] [nt:FullOosvar]
        "block" [tt:block] [nt:StatementBlockInBraces]
            "block" [tt:block] [nt:StatementBlock]

