Invoke as: combine <depth> <words separated by blanks>

E.g.: % combine 4 a b c

(no errorhandling at all)
--------------------------------
#!/usr/bin/zsh

c=$1; shift

# starts with 2 anyhow
[ $c -le 2 ] && c=0 || ((c-=2))

combine() {
  c=$1; shift
  # reassemble arrays
  words=(`for i in "$@"; do [ $i = "--" ] && break; echo $i; done`)
  combinations=(`shift ${#words[*]}; shift; for i in "$@"; do echo $i; done`)
  if [ $c -ge 0 ]; then
    ((c--))
    com2=(`for i in $words; do
      for j in $combinations; do
         echo $i$j
      done
    done`)
    combine $c $words -- $com2
  else
    for j in $combinations; do
      echo $j
    done
  fi
}

combine $c "$@" -- "$@"