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assert.

assert. T
An assertion failure is signalled if the single sentence T is not an array of all 1s. Assertions are evaluated or not according to the flag controlled by 9!:34 and 9!:35 .
 

For example:
cfi=: 4 : 0 " 0                The y-th combination of i.x
 assert. 0<:y
 assert. y=<.y
 assert. y<2^x
 v=. +/\(i.x)!x
 m=. (y<v)i. 1
 (m,x) ci (y-m{0,v)
)

ci=: 4 : 0 " 1 0     
 'm n'=. x
 if. 0=m do.
  i.0
 else.
  v=. +/\ (m-1)!(1-m)}.i.-n
  k=. (v>y) i. 1
  k,(1+k)+(x-1,1+k)ci(y-k{0,v)
 end.
)

   5 cfi 6
0 1

   5 cfi 6+i.10
0 1
0 2
0 3
0 4
1 2
1 3
1 4
2 3
2 4
3 4

   (i.100) -: 100x cfi <:2^100x
1

   5 cfi 33
|assertion failure: cfi
|       y<2^x
   5 cfi 6.2
|assertion failure: cfi
|       y=<.y
   5 cfi 'a'
|domain error: cfi
|   y=    <.y


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