Fix XML indentation in Emacs mode The return value of MATCH-STRING is a string. At least on Emacs 25, the comparisons between string and character with EQUAL could never succeed, and so the cases for matching braces were never triggered. GET-TEXT-PROPERTY may return a list rather than an atom (for example, on long lines with whitespace-mode turned on), and this broke the heuristic of looking for the tag face in previous text.

fun ack (m, n) =
    if m = 0 then
        n + 1
    else if n = 0 then
        ack (m - 1, 1)
    else
        ack (m - 1, ack (m, n - 1))

fun main n = return <xml>{[ack (n, 4)]}</xml>