Constrain a "parameter" which is not used to fit

I have a little problem with a fit.
I have several points which define a graph.
I would like to fit this graph with a function.
My function depends on one variable variable called X. It depends also on one parameter called A. This parameter is a function of X of type: A = a1*X+a0
Both parameters a1 and a0 are used to fit the graph.
I can contrain the value of these parameters but I would like to contrain the value of the parameter A. For instance, I would like that 0 < A < 1 for the range of X where my function is defined (let us say between 0 and 10). The value of the parameters a0 and a1 should be compatible for all the values of X between 0 and 10.

I do not know if my problem is clear but is so, is it possible to do that?
Thanks in advance.


A is another parameter of your function. It can be free, fixed or have simple bounds, like 0 < A < 1 when fitting
See TF1::FixParameter or TF1::SetParLimits