Number of List Elements



Perhaps the function I am looking for is to be found in the help files but, if so, it has so far elluded me.  What I am trying to do is find out the number of the elements in a list.  Specifically, I want a function that returns the number of elements of a list.  Its behavior would look like:

A : [1,2,3];
B : [ [a,b], c, d+e];

num_elem(A) -> 3
num_elem(B) -> 3

I would also be interested in seeing how one would program such a function for oneself.

Thanks in advance,
Conrad