Community standards for mathematical expression trees

As @jpfairbanks has pointed out in #2, Catlab includes a `Formula` data type for representing mathematical expressions (e.g. `sin(x)` or `x^2+1`) as expression trees. Catlab is not trying to be a conventional computer algebra system like Mathematica or SymPy. Rather, this data type exists to facilitate communication with such systems.

The package [ModelingToolkit.jl](https://github.com/JuliaDiffEq/ModelingToolkit.jl) also has a data type for expressions, as do presumably [other packages](https://discourse.julialang.org/t/sympy-jl-vs-symengine-jl-vs-reduce-jl-vs/10381) in the Julia ecosystem. Is it possible to converge on a shared representation?