Iteration problem

I'm trying to work out a simple loop to produce the following results

/*
    keywords = dray aerial COMPACT tailed

    dray aerial compact tailed  0   1   2   3
    dray aerial compact         0   1   2
    dray aerial tailed          0   1   3
    dray compact tailed         0   2   3
    aerial compact tailed       1   2   3
    dray aerial                 0   1
    dray compact                0   2
    dray tailed                 0   3
    aerial compact              1   2
    aerial tailed               1   3
    compact tailed              2   3
    dray                        0
    aerial                      1
    compact                     2
    tailed                      3
*/

So something like:

n = length keywords

loop until something
{
  keys = empty array

  for ( i = 0 to n )
  {
     add to keys ( keywords[i] )
  }
  print keys

  n = length keys - 1
}

but that's far from the right answer and hence this question :)

This question has also been asked under the SO section as it is for PHP and programmers are quite logical too http://stackoverflow.com/questions/14672108/simple-iteration-formula-for-php

via Recent Questions - Mathematics - Stack Exchange http://math.stackexchange.com/questions/293549/iteration-problem