Fit with constraints using a parameter wave

Function N2_qcq(w,x) : FitFunc // quadratic cubic quadratic fit
    Wave w //  
    Variable x
   
    if( x<w[0])
         return    w[1] + w[2]*(x-w[0]) + w[6]*(x-w[0])^2;
    elseif( x <w[3] )
            return w[1]*(x-w[3])/(w[0]-w[3]) + w[4]*(x-w[0])/(w[3]-w[0]) + (w[2]-(w[4]-w[1])/(w[3]-w[0]))*(x-w[0])*(x-w[3])^2/(w[0]-w[3])^2+ (w[5]-(w[4]-w[1])/(w[3]-w[0]))*(x-w[0])^2*(x-w[3])/(w[3]-w[0])^2;
    else
                return w[4] + w[5]*(x-w[3]) + w[7]*(x-w[3])^2;
     endif
end