Rules don't work on Differentials

[AwesomeQuest]

@variables a b c(a,b)
r = @rule Differential(a)(~x) => Differential(a)(b)*Differential(b)(~x)
r(D(a)(c)) # nothing

Rules don't seem to work on Differentials as seen in the example above applying the chain rule.

[AayushSabharwal]

It needs to be phrased differently:

julia> D = Differential(a)
julia> r = @rule D(~x) => D(b)*D(~x)
julia> r(D(c))
Differential(a, 1)(c(a, b))*Differential(a, 1)(b)