From type/Positional

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method EXISTS-POS(\position)

Should return a Bool indicating whether the given position actually has a value.

From type/Range

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multi method EXISTS-POS(Range:D: int \pos)
multi method EXISTS-POS(Range:D: Int \pos)

Returns True if pos is greater than or equal to zero and lower than self.elems. Returns False otherwise.

say (6..10).EXISTS-POS(2); # OUTPUT: «True␤» 
say (6..10).EXISTS-POS(7); # OUTPUT: «False␤»

From type/Sequence

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multi method EXISTS-POS(::?CLASS:D: Int:D $idx)
multi method EXISTS-POS(::?CLASS:D: int $idx)

Returns a Bool indicating whether there is an element at position $idx in the cached sequence.

From language/subscripts

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multi method EXISTS-POS (::?CLASS:D: $index)

Expected to return a Bool indicating whether or not there is an element at position $index. This is what postcircumfix [ ] calls when invoked like @foo[42]:exists.

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.