### how to define the codomain of a symbolic function

Hi,
I'm perfoming some basic differential calculations in General Relativity.

I ~~defined ~~created a function a(t), where t is one of the chart coordinates:

a = function('a')(t)

I do not want to define how it depends on t since I'm calculating stuff like Cristoffel symbols, Einstein equations, etc and I want to keep the resulting formulas independent from the functional behaviour of a(t).

This function appears to be a complex one (i.e. I see the bar over it sometimes - complex conjugate).
This is boring since the calculations does not simplify by themselves.

Is there a way to "assume" that the codomain of the function are the Real Numbers? It would be great also to specify only a range, like (0,+\infty).

Thanks