Errors in digital notch filter function block .

How do i remove all those errors.? Any fast response really appreciated!

function filter_output = fcn(input_current, sample_instant)

persistent x_t1 y_t1 y_t2 y_t x_t2;

%persistent value are equivalent to the static variable

%initialization of persistent variables to zero value. if isempty(x_t1)

        x_t1=0;
    end

    if isempty(x_t2)
        x_t2=0;
    end

    if isempty(y_t1)
        y_t1=0;
    end

    if isempty(y_t)
        y_t=0;
    end

    if isempty(y_t2)
        y_t2=0;
    end

if (sample_instant == 0)
   y_t = (0.9975*x_t2-1.9911*x_t1+0.9975*input_current-0.9950*y_t2+1.9911*y_t1);

     y_t2 = y_t1;
     y_t1 = y_t;
     x_t2 = x_t1;
     x_t1 = input_current;
  end
end