

Arithmetic

KEY
Arithmetic is performed by built-in procedures which take as arguments  integer
expressions  and  evaluate  them.    An integer expression is a term built from
evaluable functors, integers and variables.  At the time  of  evaluation,  each
variable  in an integer expression must be bound to an integer or to an integer
expression.

Each evaluable functor stands for  an  arithmetic  operation.    The  evaluable
functors are as follows, where X and Y are integer expressions.

    X+Y         integer addition

    X-Y         integer subtraction

    X*Y         integer multiplication

    X/Y         integer division

    X mod Y     X modulo Y

    -X          unary minus

    X/\Y        bitwise conjunction

    X\/Y        bitwise disjunction

    X<<Y        bitwise left shift of X by Y places

    X>>Y        bitwise right shift of X by Y places

    [X]         (a  list  of  just  one  element) evaluates to X if X is an
                integer.  Since a quoted string is just a list of integers,
                this allows a quoted character to be used in place  of  its
                ASCII  code; e.g. "A" behaves within arithmetic expressions
                as the integer 65.

The arithmetic built-in procedures are as follows, where  X  and  Y  stand  for
arithmetic expressions, and Z for some term.  Note that this means that is only
evaluates  one  of its arguments as an integer expression (i.e. X), whereas all
the comparison predicates evaluate both their arguments.
Z is X      Integer expression X is evaluated and the result, is  unified  with
            Z. Fails if X is not an integer expression.

X =:= Y     The values of X and Y are equal.

X =\= Y     The values of X and Y are not equal.

X < Y       The value of X is less than the value of Y.

X > Y       The value of X is greater than the value of Y.

X =< Y      The value of X is less than or equal to the value of Y.

X >= Y      The value of X is greater than or equal to the value of Y.