From Mill Computing Wiki
Count leading false in or construct bool vector with leading false.
operands: like Leftv XX:i
This counts the leading false elements in a bool vector.
|Silver||E0 E1 E2 E3||1|
operands: like Lefts xx:X
Constructs a bool vector with w elements of which the leading x elements are false and the rest true.
|Silver||E0 E1 E2 E3|