Documentation for method
EXISTS-POS assembled from the following types:
multi method EXISTS-POS (::?CLASS: )
Expected to return a Bool indicating whether or not there is an element at position
$index. This is what
postcircumfix [ ] calls when invoked like
What "existence" of an element means, is up to your type.
If you don't implement this, your type will inherit the default implementation from
Any, which returns True for 0 and False for any other index - which is probably not what you want. So if checking for element existence cannot be done for your type, add an implementation that fails or dies, to avoid silently doing the wrong thing.