I am wondering how I can determine if more than one bit of a four-bit STD\_LOGIC\_VECTOR is set to '1'. 

e.g if it is "1001" or "1100" or "1111".

I am writing a program where I have to set an error signal to '1' if I get more than one control signal to my entity. The four control signals have been merged into one 4-bit STD\_LOGIC\_VECTOR and I need a smart way to determine if more than one of the bits are set.