function
(next ds &opt key )
Gets the next key in a data structure. Can be used to iterate
through the keys of a data structure in an unspecified order. Keys
are guaranteed to be seen only once per iteration if the data
structure is not mutated during iteration. If key is nil , next
returns the first key. If next returns nil , there are no more keys
to iterate through.
(next [4 5 6 ] ) # => 0
(next [4 5 6 ] 0 ) # => 1
(next [4 5 6 ] 1 ) # => 2
(next [4 5 6 ] 2 ) # => nil
# note that dictionary keys are not necessarily in the same order
# as the corresponding literal.
(next {:a 5 :b 6 :c 7 } ) # => :a
(next {:a 5 :b 6 :c 7 } :a ) # => :c
(next {:a 5 :b 6 :c 7 } :c ) # => :b
(next {:a 5 :b 6 :c 7 } :b ) # => nil