unikernel/duniverse/dune_/doc/reference/predicate-language.rst

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+Predicate Language
+==================
+
+The predicate language allows the user to define simple predicates
+(Boolean-valued functions) that Dune can evaluate. Here is a semiformal
+specification of the predicate language:
+
+.. productionlist::
+   pred : (and `pred` `pred`)
+        : (or `pred` `pred`)
+        : (not `pred`)
+        : :standard
+        : `element`
+
+The exact meaning of ``:standard`` and the nature of :token:`element` depend on
+the context.
+For example, in the case of the :doc:`/reference/dune/subdir`, an
+:token:`element` corresponds to file glob patterns.
+Another example is the user action :doc:`actions/with-accepted-exit-codes`,
+where an :token:`element` corresponds to a literal integer.