Variadic parameters

Functions support variadic parameters with the special symbols * and +.

A * parameter is optional and defaults to an empty list, while a + parameter is required and must contain at least one element.

Functions may only have up to one variadic parameter, and it must appear last in the signature.

[$how-many: items*] {
    [len: <items>]
}

[how-many: foo; bar; baz] # Outputs "3"

@lazy cannot be combined with * or +. If you need lazy evaluation, accept a single lazy parameter and decide inside the function how to realize it.