Function            sum_year_digits_dep - calculate sum of the years
                                               digits depreciation

  Syntax              double sum_year_digits_dep(double cost, double
                                     salvage, int life, int period);

  Prototype in        finance.h

  Remarks             given the cost, salvage value and life of an item,
                      sum_year_digits_dep will calculate the amount of
                      deprecitation for the given period according to the
                      sum of the years digits depreciation method. The
                      cost and salvage can be given in any unit (dollars,
                      thousands of dollars, etc.) but the life should be
                      given in depreciable periods (if you depreciate an
                      item every quarter, and the item has a life of 2
                      years, then life should be 8).

                      The cost and salvage values should be in the same
                      units. The life and period should be given in the
                      same units.

                      No error checking is performed.

  Return value        returns the amount of depreciation for the given
                      period in the same units as the cost, as per the
                      sum of the years digits depreciation method.

  Note                The macro SYD(c,s,l,p) is defined in finance.h for
                      ease of use.

  See also            accum_dep(), depreciation(),
                      double_decline_bal_dep(), straight_line_dep()

  Example             see demonum.c