Suppose I have defined the following function inside a package that can take 2 arguments.

```
f[x_,y_]:={x,y}

```

So, if the user gives `f[1,2]` then the output is `{1,2}`. But if the input is `f[1,2,3]` then the output is also `f[1,2,3]`.

I want to print an error message when the input is `f[1,2,3]`, but I can't figure out how to count the number of arguments given by the user.