Syntax Error

Hi.. i'm wondering if you can explain some of your codes to me.. especially this part..:

when s1 =>

if input = "0111" then

g <= "1000";

y <= "0000";

r <= input;

state <= s2;

elsif input = "1011" then

g <= "0100";

state <= s2;

elsif input = "1101" then

g <= "0010";

state <= s2;

elsif input = "1110" then

g <= "0001";

state <= s2;

end if;

when s2 =>

if timer = "0100" then

state <= s4;

g <= "0000";

else

timer <= timer + 1;

end if;

when s4 =>

if input = "0111" then

y <= "1000";

state <= s5;

elsif input = "1011" then

y <= "0100";

state <= s5;

elsif input = "1101" then

y <= "0010";

state <= s5;

elsif input = "1110" then

y <= "0001";

state <= s5;

end if;

when s5 =>

if timer = "0010" then

state <= s7;

y <= "0000";

else

timer <= timer + 1;

end if;

when s7 =>

if input = "0111" then

r <= "1011";

state <= s8;

elsif input = "1011" then

r <= "1101";

state <= s8;

elsif input = "1101" then

r <= "1110";

state <= s8;

elsif input = "1110" then

r <= "0111";

state <= s8;

end if;

when s8 =>

if input = "0111" then

input <= "1011";

state <= s1;

elsif input = "1011" then

input <= "1101";

state <= s1;

elsif input = "1101" then

input <= "1110";

state <= s1;

elsif input = "1110" then

input <= "0111";

state <= s1;

end if;

end case;