Multiplexer VHDL

ENTITY mux IS
PORT ( a, b, c, d : IN BIT;
s0, s1 : IN BIT;
x, : OUT BIT);
END mux;

ARCHITECTURE dataflow OF mux IS
SIGNAL select : INTEGER;
BEGIN
select <= 0 WHEN s0 = ‘0’ AND s1 = ‘0’ ELSE
1 WHEN s0 = ‘1’ AND s1 = ‘0’ ELSE
2 WHEN s0 = ‘0’ AND s1 = ‘1’ ELSE
3;
x <= a AFTER 0.5 NS WHEN select = 0 ELSE
b AFTER 0.5 NS WHEN select = 1 ELSE
c AFTER 0.5 NS WHEN select = 2 ELSE
d AFTER 0.5 NS;
END dataflow;