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[: caps a left branch of a fork as described in
Section II F. For example,
the verb p=: [: +/ + * 
applies the monad +/ to the result of the fork + *  .
[: signals error on any argument.

Caps make it possible to define a wider range of functions
as unbroken trains. For example, the maximum divided by the
product of the sum and difference would be defined by a single train,
whereas (without the use of the cap) the definition
of the maximum divided by the (monad) floor of the product
of the sum and difference would require the use of trains interrupted
by the monad. Thus:
f=: >. % + * 
g=: >. % <. @ (+ * )
2.5 f 4
_0.410256
2.5 g 4
_0.4
The cap makes possible the use of an unbroken train as follows:
h=: >. % [: <. + * 
2.5 h 4
_0.4
Since the domain of the cap is empty, it can be used
(with :) to define a function whose
monadic or dyadic case invokes an error. For example:
abs=:  : [:
res=: [: : 
res _4 0 5
valence error: res
 res _4 0 5
abs _4 0 5
4 0 5
3 res _4 0 5
2 0 2
3 abs _4 0 5
valence error: abs
 3 abs _4 0 5
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