// create "root function(TF1)" to use in fit
// ex)
// root[i] .x slew.C
// root[i] fit("slew") // using Nadeko
// root[i] hist->Fit("slew") // not using Nadeko

Double_t func_slew(Double_t *x, Double_t *par)
{
  return par[0]-par[1]/sqrt(x[0]);
}

void slew()
{
  TString name = "slew";
  Double_t (*fcn)(Double_t *,Double_t *) = func_slew;
  Double_t xmin=0;
  Double_t xmax=1;
  Int_t npar=2;
  
  TF1 *f1 = new TF1(name, fcn, xmin, xmax, npar);
  f1->SetParameters(0, 1);
  f1->SetParNames("constant", "coefficient");
}