So, here is a recipe for a diet grapefruit soda. I call it Frasca, because a) the name is close enough to that commercial product without blatantly violating copyright, and b) just call it a tribute to the designers and manufacturers of those fine Frasca general aviation flight simulators.
Step 1: Acquire Foodstuffs and Tools
1 medium grapefruit
2 cups Splenda or other suitable sweetener
SodaStream straight-up seltzer
Hand or sieve
spatula or other stirring tool