Accept objects of Symbolics.jl

[ohno]

This issue is related to #44. Please add some if statements to escape following error:

using Symbolics
@variables r
if r < 0
end
--- output ---
TypeError: non-boolean (Num) used in boolean context

Stacktrace:
 [1] top-level scope

Goal:

using Antique
H = HydrogenAtom()
r^2 * R(H, r)^2

$$4 \left( e^{ - \left|r\right|} \right)^{2} r^{2}$$