def _get_rho(self, w, arr):
lsb = <NUM_LIT:0><EOL>while not (w & arr[lsb]):<EOL><INDENT>lsb += <NUM_LIT:1><EOL><DEDENT>return lsb + <NUM_LIT:1><EOL>
Return the least signifiant bit
            O(N) in the worst case