>   Array#only
>     [5].only      -> 5
>     [1,2,3].only  -> exception

I prefer: Array#one
to be complemented by
Array#one? #{|e| predicate(e)}  true if exactly one match

and rounded out by
Array#any #=> return a random element, like Array#any?
Array#none # with addition of Array#none? {|e| predicate(e)}
Array#all #=> of questionable value

and separately
Array#include_all? (other_enumerable)