Quintus Prolog

These predicates use a standard total order when comparing terms. The standard total order is:

     
     variables @< database references @< numbers @< atoms @< compound terms

(Interpret @< as "comes before".)

Within these categories, ordering is as follows.

Variables are put in a standard order. (Roughly, the oldest variable is put first; the order is not related to the names of variables. Users should not rely on the order of variables. They should be considered implementation dependent. The ordering of variables within a sorted list, as produced by setof/3 or sort/2, shall remain constant.)
Database references are put in a standard order (the order is based roughly on the time of creation of the reference).
Numbers are put in numeric order. Where a number may be represented by an integer or a floating-point number, as in 2 and 2.0, the integer is considered to be infinitesimally smaller than its floating-point counterpart.
Atoms are put in alphabetical order according to the character set in use.
Compound terms are ordered first by arity, then by the name of the principal functor, then by the arguments (in left-to-right order).
Lists are compared as ordinary compound terms with functor ./2

For example, here is a list of terms in the standard order:

     [ X, '$ref'(123456,12), -9, 1, 1.0, fie, foe, fum, [1],
          X = Y, fie(0,2), fie(1,1) ]

The predicates for comparison of terms are described below.

T1 == T2

T1 and T2 are literally identical (in particular, variables in equivalent positions in the two terms must be identical).

T1 \== T2

T1 and T2 are not literally identical.

T1 @< T2

T1 is before term T2 in the standard order.

T1 @> T2

T1 is after term T2

T1 @=< T2

T1 is not after term T2

T1 @>= T2

T1 is not before term T2

compare(Op, T1, T2)

the result of comparing terms T1 and T2 is Op, where the possible values for Op are: