Transfer function that implements a “latching” mechanism in one of two variants, depending on the value of the parameter keep_value.

If keep_value is zero: All states start at zero. If the input to a unit is negative, set the state to low_value; if the input to a unit is positive, set the state to high_value; otherwise, copy the previous state.

If keep_value is non-zero: All states start at zero. If the input to a unit is non-zero, copy the input to the state; otherwise, copy the previous state.

parameter	default_value
keep_value	0
low_value	-1
high_value	1