SQL-Server Set-Based Running Subtotals (i.e., Row-Level Accumulators) Example 1

SET-BASED Running Subtotals in SQL
Have you ever wanted to perform a set-based running-subtotals and/or running-totals operation in SQL-Server (i.e., have row-level accumulators store a running total or running subtotal?) Want the solution to not use cursors? Don't have access to SQL-Server 2012 or newer yet?

Here I present a cursorless method of implementing such functionality in a