my &helper = -> @prev, @rest { ... }
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Too much syntax! You can use lexically scoped subs in P6 -- you don't have to use pointy-blocks for everything:

    my sub helper (@prev, @rest) { ... }
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Now I'm wondering if its possible to get rid of the "if" statment using multi-subs; And after that if its possible to get rid of the indexing into @rest. Something like

sub outer(*@vals)
{
   my multi sub helper (@prev) {
      take @prev
   }
   my multi sub helper (@prev, $current, *@rest) {
      $current.map: { helper [@prev, $_], *@rest }
   }
   gather { helper [], *@vals }
}
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This is totally untested -- I haven't been able to install pugs yet.

--Dave.