The remaining variable, f , can be determined solely from the derivation's presupposition set using the old AI planning heuristic "use existing objects" (Sacerdoti, 1977) to avoid inventing new objects when others are already available.
we can unify alts(f, λx.browser(x) ∧ ¬f (x)) and alts(netscape, λx.browser(x) ∧ ¬f (x)), discovering that f , the figure, is netscape.