Jan Burse, created Sep 15. 2018
* Type testing allows checking terms for their type without an
* attempt to instantiate. These predicates are therefore meta-logical.
* The basic data types of the ISO Prolog core standard are variable,
* atom, integer, float and compound. The Jekejeke Prolog system adds to
* these data types the data types reference and decimal.
* ?- callable(p(X,Y)).
* ?- callable(1).
* We find elementary test predicates such as var/1, atom/1, integer/1,
* float/1 and compound/1. For the Jekejeke Prolog specific data types
* we find the test predicates reference/1, decimal/1 float32/1 and
* float64/1. We find also test predicates that group different data
* types together such as nonvar/1, atomic/1, number/1 and callable/1.
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* integer(X): [ISO 8.3.3]
* The predicate succeeds when X is an integer.
* float(X): [ISO 8.3.4]
* The predicate succeeds when X is a float.
* atom(X): [ISO 8.3.2]
* The predicate succeeds when X is an atom.
* compound(X): [ISO 8.3.6]
* The predicate succeeds when X is a compound.
* The predicate succeeds when X is a reference.
* The predicate succeeds when X is a decimal.
* number(X): [ISO 8.3.8]
* The predicate succeeds when X is a number, i.e. an integer, a float or a decimal.
* callable(X): [TC2 8.3.9]
* The predicate succeeds when X is callable, i.e. an atom or a compound.
* atomic(X): [ISO 8.3.5]
* The predicate succeeds when X is a constant, i.e. an atom, a number
* or a reference.
* var(X): [ISO 8.3.1]
* The predicate succeeds when X is a variable.
% already defined in member
* nonvar(X): [ISO 8.3.7]
* The predicate succeeds when X is not a variable, i.e. atomic or compound.
% already defined in member
* The predicate succeeds when X is a float32.
* The predicate succeeds when X is a float64.