clear persistent in function simulink

hallo,

i need to clear persistent in simulink function when ytyt==15 but it say this:

Function 'clear' is not supported for code generation and the coder.extrinsic directive cannot be applied to 'clear'. Rewrite your code so it does not use 'clear'.

so how i can do it ?

function y = fcn(u)
persistent ytyt;
if isempty(ytyt)
    ytyt=u
end
ytyt=ytyt+1
if ytyt == 15
    clear ytyt
end
y = ytyt
end