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 last(.words[.i]) != first(.words[.i+1]): break = false
}

# .amb expects 2 or more arguments
val .amb = fn(...[2 .. -1] .words) if(.alljoin(.words): join " ", .words)

writeln join "\n", filter mapX .amb, .wordsets...