val wordsets = [
    fw/the that a/,
    fw/frog elephant thing/,
    fw/walked treaded grows/,
    fw/slowly quickly/,
]

val alljoin = fn(words) { for[=true] i of len(words)-1 {
    if words[i][-1] != words[i+1][1]: break val=false
}}

# amb expects 2 or more arguments
val amb = fn(words...[2..]) { if alljoin(words) { join words, delim=" " } }

writeln join(mapX(wordsets..., by=amb) -> filter, delim="\n")