--
 Decrement--unary                                (Mathematical)
------------------------------------------------------------------------------
 Syntax

     --<idVar>       (prefix decrement)
     <idVar>--       (postfix decrement)

 Type

     Date, numeric

 Operands

     <idVar> is any valid CA-Clipper identifier, including a field
     variable.  If <idVar> is a field, you must reference it prefaced with an
     alias or declare it with the FIELD statement.

     In addition, you must initialize <idVar> to a value before performing
     the decrement operation, and it must be either numeric or date data
     type.

 Description

     The decrement operator (--) decreases the value of its operand by one.
     This operator subtracts one from the value of <idVar> and assigns the
     new value to <idVar>.

     The -- operator can appear before or after <idVar>.  Specifying the
     operator before <idVar> decrements and assigns the value before the
     value is used.  This is called prefix notation, and it is the most
     common usage.  Specifying the operator after <idVar> decrements and
     assigns the value after it is used.  This is postfix notation.  Stated
     differently, postfix notation delays the assignment portion of the
     operation until the rest of the expression is evaluated, and prefix
     notation gives the assignment precedence over all other operations in
     the expression.

     If the reference to <idVar> is ambiguous (i.e., not declared at compile
     time and not explicitly qualified with an alias), <idVar> is always
     assumed to be MEMVAR.  You can assign field variables by declaring the
     field variable name in a FIELD statement or by referring to the field
     name prefaced by the FIELD-> alias or by the name of the work area.

 Examples

     .  In this example of the postfix decrement operator the
        assignment takes place before the original variable is decremented,
        thus the two values are not the same when queried:

        nValue := 1
        nNewValue := nValue--
        ? nNewValue                  // Result:   1
        ? nValue                     // Result:   0

     .  In this example, the prefix decrement operator decreases the
        first operand of the multiplication by one, making its value 9.
        Then, the assignment of this new value to the nValue variable takes
        place before the rest of the expression is evaluated.  Thus, the new
        value is used to perform the multiplication operation, and the result
        of 9 * 9 is 81.

        nValue := 10
        ? --nValue * nValue          // Result:  81
        ? nValue                     // Result:   9