DSL EXPRESSION:
$ENV["FOO"][2] = "bar"


AST:
"block" [tt:block] [nt:StatementBlock]
    "=" [tt:equals] [nt:Assignment]
        "[]" [tt:[]] [nt:ArrayOrMapIndexAccess]
            "[]" [tt:[]] [nt:ArrayOrMapIndexAccess]
                "$ENV" [tt:field_name] [nt:DirectFieldValue]
                ""FOO"" [tt:string_literal] [nt:string_literal]
            "2" [tt:int_literal] [nt:int_literal]
        ""bar"" [tt:string_literal] [nt:string_literal]

