Add spec checking that Set#first is the first item yielded by Set#each
Fixed typo in Set#eql? spec
Vector#fetch passes missing index to default block (like Ruby's built-in Array)
SortedSet#fetch passes missing index to default block (like Ruby's built-in Array)
Added comment on types which can be inserted into a SortedSet in Yardoc class summary
Tweaked whitespace in Hamster.iterate code sample
Added spec for Vector#group_by
Not all SortedSets have a comparator
SortedSets can be marshalled/unmarshalled