Function            straight_line_dep - calculate straight line
                                            depreciation

  Syntax              double straight_line_dep(double cost, double
                                               salvage, int life);

  Prototype in        finance.h

  Remarks             given the cost, salvage value and life of an item,
                      straight_line_dep will calculate the periodic
                      deprecitation according to the straight line method
                      of depreciation. 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.

                      No error checking is performed.

  Return value        returns the periodic depreciation in the same units
                      as the cost, as per the straight line depreciation
                      method.

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

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

  Example             see demonum.c