Section: Inspection Functions
x, or in a subindex
expression.  The syntax for its use is either
y = numel(x)
or
y = numel(x,varargin)
Generally, numel returns prod(size(x)), the number of total
elements in x.  However, you can specify a number of indexing
expressions for varagin such as index1, index2, ..., indexm.
In that case, the output of numel is 
prod(size(x(index1,...,indexm))).
4 x 4 x 3 matrix, the length is 4, not 48, as you 
might expect, but numel is 48.
--> x = rand(4,4,3); --> length(x) ans = 4 --> numel(x) ans = 48
Here is an example of using numel with indexing expressions.
--> numel(x,1:3,1:2,2) ans = 6