bell=: ([: +/\ (,~ {:))&.>@:{:

   ,. bell^:(<5) <1
+--------------+
|1             |
+--------------+
|1 2           |
+--------------+
|2 3 5         |
+--------------+
|5 7 10 15     |
+--------------+
|15 20 27 37 52|
+--------------+

   {.&> bell^:(<15) <1
1 1 2 5 15 52 203 877 4140 21147 115975 678570 4213597 27644437 190899322

   {:>bell^:49<1x
185724268771078270438257767181908917499221852770