⌾fn1 ⌾ fn2; fn2 must be customisableProvides a custom internal method for the customisable function fn2. Functions where there is internal processing (for example, the comparison done within the sorting algorithm) may allow you to provide your own method instead. The requirements for fn1 vary depending on what function fn2 is. The result will be a modified version of fn2, syntactically identical (i.e. same valence, rank etc).
The following functions in the standard set are customisable:
⍋/⍒: the sort functions
fn1 should take.
The ContextException is thrown if fn2 is not customisable, or if fn1 is not a suitable custom function.