(comb=
  bvar combination combinations list m n pat pvar var
.     !arg:(?m.?n)
    & ( pat
      =   ?
        & !combinations (.!combination):?combinations
        & ~
      )
    & :?list:?combination:?combinations
    &   whl
      ' ( !m+-1:~<0:?m
        & chu$(utf$a+!m):?var
        & glf$('(%@?.$var)):(=?pvar)
        & '(? ()$pvar ()$pat):(=?pat)
        & glf$('(!.$var)):(=?bvar)
        & (   '$combination:(=)
            & '$bvar:(=?combination)
          | '($bvar ()$combination):(=?combination)
          )
        )
    &   whl
      ' (!n+-1:~<0:?n&!n !list:?list)
    & !list:!pat
  | !combinations);