Sum of list (or matrix) elements transformed by a function

The count command adds the values of a function applied to the elements of a list or matrix.

count takes two arguments:
- f, a real-valued or boolean-valued function.
- L, a list or a matrix.
count(f,L) returns the sum of f(x) for all elements x in L.
If f is a boolean-valued function, this is just the number of elements for which the boolean is true.

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

147

because 2+12+45+3+7+78=147.

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

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

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

count(x->x^2,[3,5,1])

Indeed, 3²+5²+1¹=35.

count(id,[3,5,1])

Indeed, id is the identity functions and 3+5+1=9.

count(1,[3,5,1])

Indeed, 1 is the constant function equal to 1 and 1+1+1=3.