# [isabelle] what the meaning of the set's operation `` ?

The following are the part of context in dfs.thy.
----------------------------------------------------------------
subsection "Definition of Graphs"
typedecl node
types graph = "(node * node) list"
consts
nexts :: "[graph, node] \<Rightarrow> node list"
primrec
"nexts [] n = []"
"nexts (e#es) n = (if fst e = n then snd e # nexts es n else nexts es n)"
definition
nextss :: "[graph, node list] \<Rightarrow> node set" where
"nextss g xs = set g `` set xs"
-----------------------------------------------------------------------
I don't know the meaning of the operation `` in the "nextss g xs = set g `` set xs"?
yucy

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