Verbs are taken from Power Set where also more information on this subject is to be found.
Collected Definitions
ps0 =: #:@i.@(2&^)@# <@#"1 _ ]
ps1a=: 3 : 'if. 0=#y do. ,<0#y else. (<{.y) (],,&.>) ps1a }.y end.'
ps1b=: ,@<@(0&#) ` (<@{. (],,&.>) $:@}.) @. (0<#)
ps2 =: , @ ((],,&.>)/) @ (<"_1 , <@(0&#))
ps3 =: (i.@>:@# comb&.> #) {&.> <@]
ps4=: 3 : '(}:,;&.>@{:) (}:, [:({.,<@(i.@#,.&.>])@}.)[:,&.>/\.>@{:)^:y(y$<i.0 0)<@,<i.1 0'
ps4a=: ([:ps4 #) {&.> < NB. for nouns which are not an integer atom.
comb=: 4 : 0 NB. All size x combinations of i.y
k=. i.>:d=.y-x
z=. (d$<i.0 0),<i.1 0
for. i.x do. z=. k ,.&.> ,&.>/\. >:&.> z end.
; z
)
Relative performance on i.20