Fix subtyping

Ideally, we should have `diff::Union{Differential,Nothing}` in the 
future.

Author: HarrisonGrodin

README.md

@@ -182,8 +182,7 @@ context-aware single variable of the IR. Its fields are described as follows:
 ### Operations
 
 Operations are the basic composition of variables and puts together the pieces
-with a function. The `~` function denotes equality
-between the arguments.
+with a function. The `~` function denotes equality between the arguments.
 
 ### Differentials
 

src/differentials.jl

@@ -1,5 +1,5 @@
-struct Differential
-    x::Union{Variable,Operation}
+struct Differential <: Expression
+    x::Expression
     order::Int
 end
 Differential(x) = Differential(x,1)

src/variables.jl

@@ -3,7 +3,7 @@ mutable struct Variable <: Expression
     value
     value_type::DataType
     subtype::Symbol
-    diff::Union{Differential,Nothing}
+    diff::Union{Expression,Nothing}  # FIXME
     dependents::Vector{Variable}
     description::String
     flow::Bool