make it work even if promote_symtype is not defined

Author: shashi

src/polyform.jl

@@ -229,8 +229,12 @@ Expand expressions by distributing multiplication over addition, e.g.,
 multivariate polynomials implementation.
 `variable_type` can be any subtype of `MultivariatePolynomials.AbstractVariable`.
 """
-expand(expr) = Postwalk(identity)(PolyForm(expr, Fs=Union{typeof(+), typeof(*), typeof(^)}, recurse=true))
+expand(expr) = unpolyize(PolyForm(expr, Fs=Union{typeof(+), typeof(*), typeof(^)}, recurse=true))
 
+function unpolyize(x)
+    simterm(x, f, args; kw...) = similarterm(x, f, args, symtype(x); kw...)
+    Postwalk(identity, similarterm=simterm)(x)
+end
 
 ## Rational Polynomial form with Div
 
@@ -284,7 +288,7 @@ function simplify_fractions(x; polyform=false)
 
     expr = Postwalk(sdiv ∘ quick_cancel)(Postwalk(add_with_div)(x))
 
-    polyform ? expr : Postwalk(identity)(expr)
+    polyform ? expr : unpolyize(expr)
 end
 
 function add_with_div(x, flatten=true)