LIBRARY ieee ;
USE ieee.math_real.ALL ;
USE ieee.std_logic_1164.ALL ;
USE ieee.std_logic_unsigned.ALL;
USE ieee.numeric_std.ALL ; -- use this above scope of **
USE ieee.std_logic_arith.ALL ; -- **
USE ieee.numeric_bit.ALL ;

VARIABLE nlut_idx : INTEGER ;

SUBTYPE slv_bit IS std_logic ; -- notice slv_bit is in the singular
TYPE slv_bits IS ARRAY ( 0 TO 1) OF slv_bit ; -- notice slv_bits is in the plural

TYPE test_bits_array IS ARRAY ( 0 TO 8) OF slv_bits ; -- 18 bits = 9 x 2-bits
VARIABLE test_bits : test_bits_array ;

nlut_idx := to_integer( ( test_bits( 8 DOWNTO 0))) ;