argn — Returns the number of input/output arguments in a function call
[lhs [,rhs] ]=argn() lhs=argn(1) rhs=argn(2)
This function is used inside a function definition.
It gives the number of actual inputs arguments (rhs)
and output arguments (lhs) passed to the function when the function is
called. It is usually used in function definitions to deal with
optional arguments.