Hello, I am trying to find the point of intersection between a TGraph and an analytic TF1. I have an example from Rene where he uses a C-style function to create a TF1 from a TGraph object, and I am trying to use this approach, but I am getting an error when trying to define the object to find a difference between the TF1 defined the normal way and the TF1 defined from the TGraph.

ROOT Version: 6.20/04

Here is a minimal example using a simple sin function:

```
using namespace std;
TGraph* trueStress;
double myfunc(double *x, double *){ return trueStress->Eval(x[0]);}
void minimalProblem(){
//populate TGraph with a sin function
Double_t x[100], y[100];
Int_t m = 20;
for (Int_t i=0;i<m;i++) {
x[i] = i*0.1;
y[i] = 10*sin(x[i]+0.2);
}
trueStress = new TGraph(m,x,y);
trueStress->Draw();
//simple TF1 of a line
TF1* line = new TF1("line","10*x");
gROOT->GetListOfFunctions()->Add(line);
//verify that line and sin function intersect somewhere, want to find the point of intersection
line->Draw("same");
//create a TF1 from my TGraph using C style function (from Rene Brun example)
TCanvas* c4 = new TCanvas("c4");
TF1* f1 = new TF1("f1",myfunc,0,2);
gROOT->GetListOfFunctions()->Add(f1);
f1->Draw();
//create new TF1 to find intersection of two other TF1s
TF1* diff = new TF1("diff", Form("abs(%s-%s)",f1->GetName(),line->GetName()),0,0.2);
}
```

The errors I get are:

```
input_line_111:2:77: error: use of undeclared identifier 'f1'
Double_t TFormula____id5398377958029293329(Double_t *x){ return TMath::Abs({f1}-(10*x[0])) ; }
^
input_line_112:2:80: error: expected ')'
Double_t TFormula____id5398377958029293329(Double_t *x){ return TMath::Abs({f1}-(10*x[0])) ; }
^
input_line_112:2:75: note: to match this '('
Double_t TFormula____id5398377958029293329(Double_t *x){ return TMath::Abs({f1}-(10*x[0])) ; }
^
Error in <prepareMethod>: Can't compile function TFormula____id5398377958029293329 prototype with arguments Double_t*
Error in <TFormula::InputFormulaIntoCling>: Error compiling formula expression in Cling
Error in <TFormula::ProcessFormula>: "f1" has not been matched in the formula expression
Error in <TFormula::ProcessFormula>: Formula "abs(f1-(10*x))" is invalid !
```

Thank you for any assistance!