When I calculate an integral of the form:

Integrate[1/((f(a)+I*x)*(f(b)+I*x)),{x,-Infinity, Infinity}],

where I is the imaginary unit. How can I assume that f(a)

is a positive function for all values of its argument? I am not

interested in changing the function to a constant and then

assuming the constants are >0 since the real problem is

complicated and I need to keep f(a) a function.