regex prefix_circumfix_meta_operator:reduce (--> List_prefix) {
    $<s> = (
        '['
        [
        | <op=infix> ']' ['«'|<?>]
        | <op=infix_prefix_meta_operator> ']' ['«'|<?>]
        | <op=infix_circumfix_meta_operator> ']' ['«'|<?>]
        | \\<op=infix> ']' ['«'|<?>]
        | \\<op=infix_prefix_meta_operator> ']' ['«'|<?>]
        | \\<op=infix_circumfix_meta_operator> ']' ['«'|<?>]
        ]
    ) <?before \s | '(' >
...