Location 3.0 and later provide a location_api_variant() function to aid in allowing add-on modules to support multiple releases, even if there are incompatible changes.

### API variant 1

Start of API tracking. Based on Location 3.0.

### API variant 2

location_diff() renamed to location_calc_difference() because diff.module was trying to invoke it as a hook.

Before: Call location_diff().

Now: Call location_calc_difference().