Dec procedure

  DECLARATION:  Dec(var x [ ; n: LongInt] )

      PURPOSE:  Decrements a variable.

         UNIT:  System

      REMARKS:  x is an ordinal type variable, and n is an integer
                type expression.  x is decremented by 1, or by n if n is
                specified; ie: Dec(x) corresponds to x := x - 1
                               Dec(x,n) corresponds to x := x - n

                Dec generates optimized code and is useful for loops.