Functions can be used in SymbolicBFI

I am trying to solve nonlinear problems using the script below.
I am starting with one-dimensioal problem.

func = lambda x: x**2
a = BilinearForm(fes, symmetric=True)
a += SymbolicBFI(v * func(u))
a += - v * x * dx
c = Preconditioner(a, type="bddc")

res = gfu.vec.CreateVector()
du = gfu.vec.CreateVector()
for niter in range(10):
	a.Apply(gfu.vec, res)
	solvers.CG(,, mat=a.mat, pre=c.mat, printrates='', tol=1e-10, maxsteps=10000) -= du

It seems working. I obtained the solusion as u = \sqrt{x}.

What are the functions that can be used in SymboliBFIs?
I want to use an interploated function from the measured data.
I tried with BSpline CoefficientFunction , but could not find a way to work with it.