the j solution to the 100 doors problem in rosetta code is ~:/ (100 $ - {. 1:)"0 >:i.100
>: i. 100 means 'make a list of numbers from 1 to 100'
(100 $ - {. 1:)"0 >: i. 100 means 'make a list of 100 lists, where each position represents an integer, and make every nth position 1'
~:/ (100 $ - {. 1:)"0 >: i. 100 means 'turn on only the numbers that have a one in their column only once' which are the squares.
but the verb train (100 $ - {. 1:) has me mystified... What does 1: mean? I found the : verb in nuvoc, but I did not see this specific application there. How are the verbs grouped?