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# Box - monad <

So far you have dealt with atoms that are either numeric or character. The monad < (box) introduces a new type of atom called boxed. The monad < applies to any noun and returns an atom that is a box which contains the argument.

An array is either a numeric array that contains numbers, or a literal array that contains characters, or a boxed array that contains boxes. Arrays of numbers and characters are referred to as open to distinguish them from boxed arrays.
```   b =. < 2 3 4
\$b		NB. an atom has empty shape
```
A boxed array is displayed in a box.
```   b
+-----+
|2 3 4|
+-----+
c =. < 4 7 9
d =. b , c	NB. append
d
+-----+-----+
|2 3 4|4 7 9|
+-----+-----+
\$d		NB. list with shape 2
2
(< 2 3 4) , < 4 7 9
+-----+-----+
|2 3 4|4 7 9|
+-----+-----+```
Arrays of different types (numeric, character, and boxed) cannot be appended to one another.
```   'a' , 3
|domain error
|   'a'    ,3
3 , < 2 3 4
|domain error
|   3    ,<2 3 4```
Boxed arrays are of the same type and can be appended no matter what they contain.
```   (< 'abc') , < 4 5 6
+---+-----+
|abc|4 5 6|
+---+-----+
(< 2 3 \$ 'abcdef') , (< i. 3 4) , < 23
+---+---------+--+
|abc|0 1  2  3|23|
|def|4 5  6  7|  |
|   |8 9 10 11|  |
+---+---------+--+```

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