DSL EXPRESSION:
$z = $a["index"]


AST:
"block" [tt:block] [nt:StatementBlock]
    "=" [tt:equals] [nt:Assignment]
        "$z" [tt:field_name] [nt:DirectFieldValue]
        "[]" [tt:[]] [nt:ArrayOrMapIndexAccess]
            "$a" [tt:field_name] [nt:DirectFieldValue]
            ""index"" [tt:string_literal] [nt:string_literal]

