Threshold Model in Roofit

Dear Experts,

I need to fit a so-called “threshold model” where my PDF changes functional form at a boundary, i.e.

PDF = f(x) if x < a

PDF = g(x) if x > a

Is there a way to construct such a PDF in Roofit? I need to fit both ranges simultaneously, to ensure that the function is continuous at x = a.

Thanks in Advance,