treat 0-arg function calls as variables in ordering

Author: shashi

src/simplify.jl

@@ -79,6 +79,12 @@ end
 <ₑ(a::T, b::S) where {T, S} = T===S ? isless(a, b) : nameof(T) < nameof(S)
 
 function <ₑ(a::Term, b::Term)
+    if arglength(a) === 0
+        return operation(a) <ₑ b
+    elseif arglength(b) === 0
+        return a <ₑ operation(b)
+    end
+
     if 0 < arglength(a) <= 2 && 0 < arglength(b) <= 2
         # e.g. a < sin(a) < b ^ 2 < b
         @goto compare_args

test/order.jl

@@ -50,3 +50,10 @@ end
         end
     end
 end
+
+@testset "0-arg variable calls ordering" begin
+    @syms z ρ
+
+    @test -1z() <ₑ ρ()
+    @test !(ρ() <ₑ -1z())
+end