function [u,v]=f(x,y)
  [lhs,rhs]=argn()
  if rhs <= 0 then error('at least one argument must be given');end 
  if rhs <= 1 then y=2;end 
  if lhs == 2 then 
    u=x; v=y;
  else
     u=x+y;
  end
endfunction

[u,v]=f(4)