These models are to cover the special cases where `append` can be used
with a second argument which is a string followed by `...`, and `copy`
can be used with a second argument which is a string. In this case the
taint is carried by the whole string, rather than in array elements.
Although the documentation makes them look variadic (and generic), they
are actually special-cased in the compiler. Like all built-in functions
they don't have a signature type, but the type of `min(a, b, c)` is
`func(int, int, int) int` and not `func(int, ...int) int`.
Go doesn't allow open-ended ranges for argument indices in
models-as-data specifications (though Ruby and Python do), so I've used
`1..1000`.