fix: issue with varying order of fields

Author: MilesCranmer

src/ParametricExpression.jl

@@ -56,7 +56,11 @@ end
 
 An expression to store parameters for a tree
 """
-struct ParametricExpression{T,N<:ParametricNode{T},D<:NamedTuple} <: AbstractExpression{T,N}
+struct ParametricExpression{
+    T,
+    N<:ParametricNode{T},
+    D<:NamedTuple{(:operators, :variable_names, :parameters, :parameter_names)},
+} <: AbstractExpression{T,N}
     tree::N
     metadata::Metadata{D}