The J and K solutions to the recent "relation challenge"

problem make use of the monad ,/ (comma slash). This is

a neat little function that collapses the leading two

dimensions of an array. That is, if $x is n,r,c then

$,/x is (n*r),c.

,/ is computed in J in linear time. A more naive

implementation can take time of order n^2, by explicitly

doing the n-1 catenations. Whichever implementation is

used can be detected by timing on arguments with n=1000.