Question: Can an arbitrary function expression be specified to be treated as real valued?

Suppose I specify a metric in say the differential geometry package or Physics package that has an arbitary function f(x,y) of some of the coordinates as a component. Can I specify Maple to treat the function as real valued when Maple is asked to compute curvature etc for the metric? In some examples I have performed Maple sometimes returns expressions in f(x,y) for components that are in fact zero once one treats f(x,y) as real valued. It would be preferable to have Maple actually return '0' in such cases.

I tried the 'assume' command but Maple complained it was being applied to a protected name and I tried using

use RealDomain in simplify (5) end use

after Maple's computations, where (5) lablled Maple's output, but Maple didn't perform any simplification.