When referring to keyboard characters, printing characters are written thus: a, while control characters are written like this: ^A. Thus ^C is the character you get by holding down the CTL key while you type c. Finally, the special control characters carriage-return, line-feed and space are often abbreviated to RET, LFD and SPC respectively.
When introducing a built-in predicate, we shall present its usage with a mode spec which has the form name(arg, ..., arg) where each arg denotes how that argument should be instantiated in goals, and has one of the following forms:
In the context of some directives, we shall need the following notation:
Predicates in Prolog are distinguished by their name and their
arity. The notation
name/arity is therefore used
when it is necessary to refer to a predicate unambiguously; e.g.
concatenate/3 specifies the predicate which is named
"concatenate" and which takes 3 arguments.
More generaly, a predicate spec may be
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