The is_x function

function is_x ( s : std_ulogic ) return boolean;
function is_x ( s : std_ulogic_vector ) return boolean;
function is_x ( s : std_logic_vector ) return boolean;

For a std_ulogic or std_logic argument, this function returns true if the value is one of the undefined values ( 'U', 'X', 'Z', 'W', '-'). It returns false otherwise.

If the argument is a vector, the function returns true if any of the elements in the would have is_x true.