Trains


L and R are arbitrary arrays. 
This clever idea from the designers of J is called Trains where a parenthesized sequence of functions (which normally would signal a SYNTAX ERROR) can be interpreted as per the above descriptions. Note that the spacing between functions is for visual purposes only — it has no effect on the interpretation.
For example,
(,⍎)'2+3'
←→ '2+3',⍎'2+3'
←→ '2+3',5
2+3 5
avg←(+/ ÷ ⍴) defines a function that computes the average of a numeric vector.
avg 1 2 3 4
←→ (+/ ÷ ⍴) 1 2 3 4
←→ (+/1 2 3 4) ÷ ⍴1 2 3 4
←→ 10 ÷ ,4
2.5
Longer Trains are defined as follows:
(e f g h) ←→ (e (f g h))
(d e f g h) ←→ (d e (f g h))
and in general
Even length: (a b c ...) ←→ (a (b c ...))
Odd length: (a b c ...) ←→ (a b (c ...))
For more applications of this concept, see the discussion in the Learning J manual.