string mlfile = "", -- eg story.txt
mltxt = iff(length(mlfile)?join(read_lines(mlfile),"\n"):"""
<name> went for a walk in the park. <he or she>
found a <noun>. <name> decided to take it home.
""")

sequence strings = {}, replacements = {}
integer startpos, endpos=1
while 1 do
    startpos = find('<',mltxt,endpos)
    if startpos=0 then exit end if
    endpos = find('>',mltxt,startpos)
    if endpos=0 then ?"missing >" abort(0) end if
    string s = mltxt[startpos..endpos]
    if not find(s,strings) then
        strings = append(strings,s)
        replacements = append(replacements,prompt_string(sprintf("Enter replacement for %s:",{s})))
    end if
end while
puts(1,substitute_all(mltxt,strings,replacements))