code: def sum(context, key, value, multiplier=1):
      
      if key not in context.dicts[0]:
          context.dicts[0][key] = 0
      context.dicts[0][key] += value * multiplier
      return ''
docs: Adds the given value to the total value currently held in ``key``.
  
      Use the multiplier if you want to turn a positive value into a negative
      and actually substract from the current total sum.
  
      Usage::
  
          {% sum "MY_TOTAL" 42 -1 %}
          {{ MY_TOTAL }}