```cvec ← {larg} (fn ##.big) rarg              ⍝ Arithmetic on large integers.

Big performs integer arithmetic using a primitive operand function chosen from:

--  ------  -------
-   sub     negate
×   mult    signum
÷   div
|   mod     abs
<   lt
≤   le
=   eq
≥   ge
>   gt
≠   ne

The  arguments may be either numeric or for larger numbers, the character vector
representation of a number. In each case, the result is a character vector.

The operator was developed by Maria Wells, with a multiply function from John R.
Clark.

Examples:

2 ×big 3                              ⍝ numb × numb
6
'2' ×big 3                            ⍝ char × numb
6
'2' ×big '3'                          ⍝ char × char
6
'6'=2 ×big 3                          ⍝ result is char.
1

'¯7000000000000000000'+big 18         ⍝ addition to large number.
¯6999999999999999982

'23000000000000000000'-big 1          ⍝ subtract  ..      ..
22999999999999999999

'199999999999999999999999' ×big 2     ⍝ multiply  ..      ..
399999999999999999999998

'19999999999' <big '20000000000'      ⍝ less than ..      ..
1

×big '¯20000000000000'                ⍝ signum    ..      ..
¯1

|big '¯199999999999999999999999'      ⍝ abs value ..      ..
199999999999999999999999

↑×big/64⍴2                            ⍝ 2*64
18446744073709551616

↑×big/⍳100                              ⍝ factorial 100.
93326215443944152681699238856266700490715968264381621468592963895217599993229915
60894146397615651828625369792082722375825118521091686400000000000000000000
0000