### 2.42.27 Count the elements of a matrix verifying a property : `count`

`count` takes as arguments : a real function `f` and a real matrix `A` of dimension `p*q` (resp a list `l` of size `n`).
`count` returns `f(A[0,0])+..f(A[p-1,q-1])` (resp `f(l[0])+..f(l[n-1])`)
Hence, if `f` is a boolean function, `count` returns the number of elements of the matrix `A` (resp the list `l`) verifying the property `f`.
Input :

`count(x->x,[[2,12],[45,3],[7,78]])`

Output :

`147`

indeed: 2+12+45+3+7+78=147.
Input :

`count(x->x<10,[[2,12],[45,3],[7,78]])`

Output :

`3`