Fix missing constructorof

Author: MilesCranmer

src/symbolic_dimensions.jl

@@ -100,6 +100,7 @@ Base.propertynames(::AbstractSymbolicDimensions) = ALL_SYMBOLS
 Base.getindex(d::AbstractSymbolicDimensions, k::Symbol) = getproperty(d, k)
 constructorof(::Type{<:SymbolicDimensions}) = SymbolicDimensions
 constructorof(::Type{<:SymbolicDimensionsSingleton}) = SymbolicDimensionsSingleton
+constructorof(::Type{<:SymbolicDimensionsSingleton{R}}) where {R} = SymbolicDimensionsSingleton{R}
 with_type_parameters(::Type{<:SymbolicDimensions}, ::Type{R}) where {R} = SymbolicDimensions{R}
 with_type_parameters(::Type{<:SymbolicDimensionsSingleton}, ::Type{R}) where {R} = SymbolicDimensionsSingleton{R}
 nzdims(d::SymbolicDimensions) = getfield(d, :nzdims)