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docs/md/builtins.md

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+@defgroup  builtins  YAP Core Built-ins                  
+
+@brief This chapter describes the core built-in predicates  that control the execution of
+Prolog programs, provide fundamental functionality such as termm manipulation or arithmetic, and support interaction with external
+resources,
+
+Many of the predicates described here have been standardised by the International Standard Organization.
+ The corresponding standartised subset of Prolog also known as ISO-Prolog.                                                                                             
+In the description of the arguments of predicates the following
+notation will be used:
+
++ a preceding plus sign will denote an argument as an "input
+argument" - the argument is read, not written, and it cannot be a free variable at the time of the call;
++ a preceding minus sign will denote an "output argument";
++ an argument with no preceding symbol can be used in both ways.