(in libkernel) the module to test if a variable has changed by a fixed amount
| generic, public :: set_interval => set_interval_int, set_interval_dbl | set the interval |
| generic, public :: set_val => set_val_int, set_val_dbl | set last value |
| generic, public :: test => test_int, test_dbl | test if value and last value differ by interval, set value -> last value |