A train of three verbs is a fork and is evaluated as follows:
(f g h) y evaluates as (f y) g (h y) x (f g h) y evaluates as (x f y) g (x h y)
A simple example of a fork is the sequence of three verbs +/ % # . The / adverb takes its left argument + and returns a verb, so there is a sequence of three verbs. Let's examine the use of this fork monadically.
(f g h) y evaluates as (f y) g (h y) (+/ % #) y evaluates as (+/ y) % (# y)
This can be read as: sum over the argument divided by the count of the argument. This is the definition of the mean or average.
(+/ % #) 5 9 12 8.66667 (+/ % #) i.9 4
You can now define mean as a fork with +/ % # .
mean =. +/ % # mean i.9 4