I started a discussion thread that may be of interest to you, basically 
discussing the possibility for directional methods to reduce confusion/clarify 
intent, should be accessible at the following URL:
http://thread.gmane.org/gmane.comp.lang.swift.evolution/19185/focus=19613

> On 24 Jun 2016, at 22:05, David Waite via swift-evolution 
> <[email protected]> wrote:
> 
> And never mind me. Not sure how I missed that in the docs that many times.
> 
> -DW
> 
>> On Jun 24, 2016, at 1:38 PM, David Waite via swift-evolution 
>> <[email protected] <mailto:[email protected]>> wrote:
>> 
>> I noticed that the new Collection index(_:offsetBy:) 
>> <http://swiftdoc.org/v3.0/protocol/Collection/#comment-func-index_offsetby_> 
>> does not define that negative offsets require a BidirectionalCollection. It 
>> also declares that negative offsets must complete in O( | offset | ) time. 
>> This differs from other methods such as distance(from:to:) 
>> <http://swiftdoc.org/v3.0/protocol/Collection/#comment-func-distance-from_to_>which
>>  indicates start <= end if not a BidirectionalCollection
>> 
>> This would preclude some data structures from strictly implementing the 
>> Collection protocol, such as singly-linked lists.
>> 
>> Would it be appropriate to indicate instead that BidirectionalCollection 
>> defines the negative offset behavior and negative offset performance 
>> constraint?
>> 
>> -DW
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