The output of a NAND with edge evaluation is only 1 if at least one input is 0 and all inputs were 1 during the last cycle.
The output is set to 1 for the duration of one cycle and must be reset to 0 at least for the duration of the next cycle before it can be set to 1 again.
A block input that is not used (x) is assigned: x = 1.
Timing diagram of a NAND with edge evaluation