Add TermCombination overloads

Author: YingboMa

src/direct.jl

@@ -164,9 +164,9 @@ let
         u = map(value, u)
         idx(i) = TermCombination(Set([Dict(i=>1)]))
         dict = Dict(u .=> idx.(1:length(u)))
-        found = []
         f = Rewriters.Prewalk(x->haskey(dict, x) ? dict[x] : x)(f)
-        _sparse(linearity_propagator(f), length(u))
+        lp = linearity_propagator(f)
+        _sparse(lp, length(u))
     end
 end
 

src/linearity.jl

@@ -78,6 +78,11 @@ function Base.:(==)(comb1::TermCombination, comb2::TermCombination)
 end
 =#
 
+# we don't care about the ordering in this case
+SymbolicUtils.:<ₑ(comb1::TermCombination, comb2::TermCombination) = true
+# to make Mul and Add work
+Base.:*(::Number, comb::TermCombination) = comb
+Base.:^(comb::TermCombination, ::Number) = comb
 function Base.:+(comb1::TermCombination, comb2::TermCombination)
     if isone(comb1) && !iszero(comb2)
         return comb2