If you enter the function with the second element of "u" less than zero, it's possible to execute the "count=count+.01" line, which may have count undefined is you never did the first part of the if block. I would think it would be a warning, not an error during editing time, but an actual error if you hit that line with count undefined during run-time.
And I don't know what an " s function" is, and I don't have the embedded toolbox so I don't know about your latter sentences.
Try it like this:
persistent count;
if isempty(count)
count = 0;
end
if u(2)>0
count=0;
else
count=count+0.01;
end
sys=u(1)+count;
