simplify_fractions fails

[matthiasbaitsch]

Recently I had a case, where simplify_fractions (and thus simplify in Symbolics.jl) fails.

@syms a::Real b::Real c::Real d::Real e::Real
simplify_fractions(1 / (4 * a * b) * (4 * (c + d) + 4 * e / 3))

(c + d) / (a*b)

From what I could figure out, the problem originates in rm_gcds(ns, ds) where 4//1 is correctly identified as GCD but somewhere down the line (in MultivariatePolynomials.jl), div(4//3, 4//1) returns 0 such the the term with e disappears from the sum.

Maybe it would be an idea, to treat a rational divisor differently. The following code fixes the issue for the example above while the tests still pass.

_divide_by_divisor(x, y) = div(x, y)

_divide_by_divisor(x::PolyForm, y::Rational) =
    PolyForm{promote_symtype(/, symtype(x), symtype(y))}(
        MP.map_coefficients(term -> term / y, x.p), x.pvar2sym, x.sym2term
    )

function rm_gcds(ns, ds)
    ns = flatten_pows(ns)
    ds = flatten_pows(ds)
    for i = 1:length(ns)
        for j = 1:length(ds)
            g = _gcd(ns[i], ds[j])
            if !_isone(g)
                ns[i] = _divide_by_divisor(ns[i], g)
                ds[j] = _divide_by_divisor(ds[j], g)
            end
        end
    end
    filter!(!_isone, ns)
    filter!(!_isone, ds)
    ns, ds
end

However, I'm sure that I miss something, my understanding of SymbolicUtils is extremely limited.

Since I am pretty new to Julia and this is the first time I try to contribute to an open source project, any advice how to proceed is warmly appreciated.