is not in the sensitivity list
Here's a sample of the errors and warnings i get when compiling through the "compile hdl simulation libraries" option..
* Error: C:\Xilinx92i\vhdl\src\simprims\simprim_VITAL_mti.vhd(22819): Internal error: EXPR, val = 495, ../../../src/vcom/checkvital.c(2054)
** Error: C:\Xilinx92i\vhdl\src\simprims\simprim_VITAL_mti.vhd(22819): Internal error: EXPR, val = 495, ../../../src/vcom/checkvital.c(2054)
** Warning: C:\Xilinx92i\vhdl\src\unisims\unisim_VITAL.vhd(44949): Synthesis Warning: Signal 'a3' is read by the process but is NOT in the sensitivity list.
** Warning: C:\Xilinx92i\vhdl\src\unisims\unisim_VITAL.vhd(44949): Synthesis Warning: Signal 'a2' is read by the process but is NOT in the sensitivity list.